rash
The adjective is derived from Middle English rash, rasch (“hasty, headstrong, rash”) [and other forms], probably from Old English *ræsċ (“rash”) (found in derivatives such as ræsċan (“to move rapidly; to flicker; to flash; to glitter; to quiver”), ræsċettan (“to crackle, sparkle”), etc.), from Proto-West Germanic *rask, *raskī, *rasku, from Proto-Germanic *raskaz, *raskuz, *raþskaz, *raþskuz (“rash; rapid”), from Proto-Indo-European *Hreth₂- (“to run, roll”). The Middle English word was probably influenced by the cognates listed below.
The adverb is derived from Middle English rashe (“quickly, rapidly”), from rash, rasch (adjective) (see above).
Cognates
* Dutch ras, rasch (“rash”)
* Middle Low German rasch (“rash”)
* Old Danish rask (“agile, nimble; fast; healthy, vigorous”) (modern Danish rask (“agile, nimble; fast; healthy, vigorous; hasty, rash”))
* Old High German reski (“impetuous, rash”) (Middle High German rasch, resch (“agile, nimble; fast; lively; healthy, vigorous”), modern German rasch, räsch, resch (“agile, nimble; fast; hasty, rash; healthy, vigorous; of food: crisp, crusty”))
* Old Norse rǫskr (“brave; healthy, vigorous”) (Icelandic röskur (“strong; healthy, vigorous”))
* Old Swedish rasker (“agile, nimble; brave; fast; vigorous”) (modern Swedish rask (“agile, nimble; fast; healthy, vigorous”))
Probably from Old French rasche, rache (“skin eruption, rash; (specifically) scabies, scurf”) (obsolete), from racher (“to scrape; to scratch”) (although this is only directly attested later than the noun), from Vulgar Latin *rāsicāre (“to scrape”), from Latin rāsus (“scraped, scratched; shaved”), the perfect passive participle of rādō (“to scrape, scratch; to shave; to rub, smooth; to brush along, graze”). Doublet of rase and raze.
Cognates
* French rash, rach (“skin eruption, rash”) (rare) (borrowed from English rash)
* Italian rasca, raschia (“skin eruption like scabies”) (obsolete) (both borrowed from Old Occitan)
* Old Occitan rasca (“skin eruption like scabies”)
From Late Middle English rashen, rassh (“to hasten, hurry, rush”) [and other forms], from Old English ræscan (“to move rapidly; to flicker; to flash; to glitter; to quiver”); see further at etymology 1.
probable cognates
* Dutch rasschen, rassen (“to hasten, hurry, rush”) (obsolete)
* Old Norse raska (“to dislodge, displace; to stir up”) (Swedish raska (“to hurry; to act carelessly or hastily”))
PIE word
 *wréh₂ds
Probably an aphetic form of arace (“to tear up by the roots; to draw away”) (obsolete), from Middle English aracen (“to remove (something) by force, pluck or pull out, tear out; to grab; to lacerate; to flay or skin (an animal); to erase, obliterate”) [and other forms], from Old French aracer, arachier (“to pull off (by physical force)”) [and other forms] (whence Anglo-Norman racher, aracher (“to pluck out, pull out”); modern French arracher (“to pull up, tear out, uproot; to extract, take out (a tooth); to peel, pull off, rip off; to buy, snap up; to fight over; to tear (oneself) away from”)), a variant of esrachier (“to eradicate, get rid of”), from Latin exrādīcāre, ērādīcāre (“to root out; to annihilate, extirpate”), from ē- (a variant of ex- (prefix meaning ‘away; out’)) + rādīx (“root of a plant”) (ultimately from Proto-Indo-European *wréh₂ds (“root”)) + -ō (suffix forming regular first-conjugation verbs).
Probably a variant of race, raze (“to demolish; to destroy, obliterate; to scrape as if with a razor”), possibly modelled after rash (etymology 5 or etymology 6). Raze is derived from Middle English rasen, racen, rase (“to scrape; to shave; to erase; to pull; to strip off; to pluck or tear out; to root out (a tree, etc.); to pull away, snatch; to pull down; to knock down; to rend, tear apart; to pick clean, strip; to cleave, slice; to sever; to lacerate; to pierce; to carve, engrave; to dig; (figuratively) to expunge, obliterate; to alter”) [and other forms], from Anglo-Norman raser, rasere, rasser, Middle French, Old French raser (“to shave; to touch lightly, graze; to level off (grain, etc.) in a measure; to demolish, tear down; to erase; to polish; to wear down”), from Vulgar Latin *raso (“to shave; to scrape; to scratch; to touch lightly, graze”), from Latin rāsus (“scraped; shaved”); see further at etymology 2.