underarm
The armpit.
Old-fashioned and now outlawed style of bowling in which the arm is not swung over the shoulder.

underassertive
Insufficiently assertive.

underassured
Insufficiently assured.

underated
Alternative spelling of underrated.

underaverage
Below average.

underbalanced
Describing a method of drilling for petroleum in which the pressure in the wellbore is kept lower than that of the formation being drilled, thus allowing fluid to enter the well as soon as it is found.

underbanked
Having inadequate access to financial services or formal banking facilities.

underbarrel
Mounted beneath the barrel of a firearm.

underbarrier
Beneath a barrier.

underbellied
Having a specified kind of underbelly.

underboated
Having an inadequate boat.

underbrained
Lacking intelligence.

underbreath
Under one's breath; inaudibly or in a barely audible manner.

underbreaths
plural of underbreath

underbred
Of inferior breeding or upbringing; vulgar, lacking in manners or finesse.
Not purebred; of an inferior strain.

underbright
Not bright enough.

underbrow
The portion of the face below the eyebrow and above the eyelid.
The lower portion of the eyebrow.

underbrows
plural of underbrow

underburdened
simple past and past participle of underburden

underbust
Below the bust.

underbusy
Insufficiently busy; not having enough to do.

undercabinet
Beneath a cabinet.

undercaffeinated
Insufficiently caffeinated.

undercanopies
plural of undercanopy

undercanopy
That live under a canopy

undercapacity
Less than capacity.

undercapitaled
Having insufficient capital.

undercapitalised
Insufficiently capitalised

undercapitalized
Lacking sufficient capital

undercarboxylated
Inadequately carboxylated.

undercarriaged
Having an undercarriage.

undercarriageless
Without an undercarriage.

undercautious
Insufficiently cautious.

undercelebrated
Not having received as much fame as one deserves.

underchallenged
simple past and past participle of underchallenge

undercharacterized
Insufficiently characterized.

undercharted
Insufficiently charted, or mapped out.

underchoreographed
Inadequately choreographed.

underchurched
Having too few churches.

undercited
Having fewer citations that would be expected

underclad
Inadequately clad; not wearing enough clothes.

underclocked
simple past and past participle of underclock

undercoiled
Insufficiently coiled

undercommented
Having inadequate documentation in the form of comments in source code.

undercompensatory
Insufficient to compensate

undercomplete
Describing a frame (in linear algebra) having a set of functions less than a basis

undercompressive
Less than normally compressive

underconceptualised
insufficiently conceptualised

underconcerned
Insufficiently concerned.

undercondensed
Insufficiently condensed

underconditioned
Inadequately conditioned.

underconfident
Insufficiently confident; lacking confidence.

underconnected
Insufficiently connected

underconsidered
Given insufficient consideration

underconsolidated
Insufficiently consolidated

underconstrained
Insufficiently constrained

undercontrolled
Insufficiently controlled

underconvicted
Of a crime: where the criminals are too infrequently convicted.

undercooked
simple past and past participle of undercook

undercooled
Insufficiently cooled
supercooled

undercoordinated
Insufficiently coordinated

undercounter
Beneath a counter.
Under-the-counter.

undercover
Performed or happening in secret.
Employed or engaged in spying or secret investigation.

undercraft
A sly trick, or subterfuge in general.
Witchcraft or sorcery.
An apprentice or journeyman.

undercreative
Insufficiently creative.

undercrewed
Inadequately crewed.

undercrustal
Synonym of subcrustal.

undercultural
Of or pertaining to an underculture.

underdamped
Possessing a damping ratio between one and zero.

underdecided
Insufficiently decided; too undecided.

underdeclared
declared to be smaller than is the case

underdefended
simple past and past participle of underdefend

underdefined
Inadequately defined.

underdense
Insufficiently dense; especially not sufficiently dense enough to form a galaxy.

underdescribed
Not described with sufficient precision.
Described further on in the same document.

underdesigned
inadequately designed

underdetermined
Having too few constraints to specify a unique solution.

underdeterrent
Serving as an inadequate deterrent.

underdiagnosed
diagnosed less frequently than its occurrence.

underdiagnostic
Relating to underdiagnosis.

underdigested
insufficiently digested

underdiscussed
That is not discussed enough.

underdispersed
Less than normally dispersed

underdispersive
Exhibiting or relating to underdispersion.

underdocumented
Having inadequate documentation.

underdoggish
Characteristic of an underdog.

underdoggy
Characteristic of an underdog.

underdominant
Relating to underdominance

underdone
insufficiently cooked; undercooked

underdramatic
Inadequately dramatic.

underdrift
Having the gearing that turns the millstone beneath the floor.

underdrifts
plural of underdrift

underdrive
Designed to turn at a slower speed than stock.

underdriven
past participle of underdrive

underdue
Not having reached the deadline yet, especially something that is owing but not yet due.

underearth
Beneath the earth; underground.

undereddened
Not dereddened.

underedited
Having undergone insufficient editing, or less than is usual.

undereducated
Insufficiently educated.

undereffective
Not effective enough.

underemphasised
Alternative spelling of underemphasized.

underemphasized
Insufficiently emphasized

underemployed
Employed in a job that offers fewer work hours than desired.

underengaged
Insufficiently engaged

underenriched
Insufficiently enriched

underenthused
Insufficiently enthused.

underentitled
Feeling or acting as though one is entitled to less than one really deserves; too retiring or self-effacing.

underequipped
Lacking sufficient equipment

underestimatable
Capable of being underestimated.

underexamined
Insufficiently examined; overlooked.

underexpanded
Less than usually expanded

underexperienced
Having had insufficient experience.

underexplored
Not explored (either physically or intellectually) to the usual degree

underexpressed
Not expressed to the usual or expected degree

underexpressing
Causing underexpression

undereye
Under the eye.

undereyes
plural of undereye

underfamiliar
Insufficiently acquainted with something.

underfeatured
Lacking in features.

underfermented
Iinsufficiently fermented.

underfinanced
Lacking sufficient financing

underfinished
Of animals reared for meat: having less fat than is desirable.

underfished
Fished less than would still be sustainable; suitable for a greater amount of fishing.

underflavored
Lacking sufficient flavor.

underfleshed
Lacking flesh or substance; skinny or scanty.

underfloor
Located beneath the floor.

underfloors
plural of underfloor

underfocused
Having insufficient focus

underfogged
Insufficiently fogged.

underfolded
Folded underneath.

underforested
Too sparsely forested.

underframed
Having an underframe.

underfucked
Not receiving enough sex.

underfulfilled
simple past and past participle of underfulfil
simple past and past participle of underfulfill

underfull
Not full enough; not filled to available capacity.

underfulls
plural of underfull

underfunded
Insufficiently funded.

underfurnished
Inadequately furnished; lacking in furniture.

underfurred
Having a layer of underfur.

underfurrow
To cover as if under a furrow; to plow in.

underglorified
Not glorified enough; not praised enough.

underglycanated
Insufficiently glycanated

underglycosylated
Insufficiently (or significantly less than normally) glycosylated

undergoverned
Subordinately governed.

undergrad
An undergraduate.

undergrade
Having the truss below the roadway, as in a deck bridge.

undergraduate
A student at a university who has not yet received a degree.

undergraduates
plural of undergraduate