I found this part fascinating. Have I been wrong about it my whole life?

(By the way, if you think that a pound or an ounce is a unit of weight, and not mass, that's a common (but potentially dangerous) misconception. Read the full discussion of this in this Frink FAQ entry.)

Why is the pound a measure of mass, not force (or currency?)

Well, in the United States, the pound has been officially defined to be a unit of mass since at least 1893 (by the Office of Standard Weights and Measures, and later by its successor, the National Bureau of Standards (NBS), which was formed in 1901. The National Bureau of Standards was renamed the National Institute of Standards and Technology (NIST) in 1988.) It has had its current value since 1959, defined as exactly 0.45359237 kilograms (also a unit of mass,) both by official notice in the Federal Register, giving it the effect of official U.S. policy, and as an official refinement by the National Bureau of Standards.

The latter document is very detailed and authoritative, and shows the very slightly different values it had in 1893 (then defined as 1/2.20462 kg, a mass), the value from 1894-1959 (then defined as 1/2.2062234 kg, also a mass, which only differs from the current exact value of 0.45359237 kg by about 1 part in 10 million.) All are quite unambiguous on this point. No standards body has, as far as I can tell, defined pound as a unit of anything other than mass, at least since 1893. (Legislation before that was ambiguous about the distinction between mass and weight.)

In the United Kingdom, the pound has been officially defined as a mass since the Weights and Measures Act of 1878, which defined it as having a very slightly smaller value (equal to approximately 0.453592338 kg.) The value of the pound was unified to its current value in all countries by 1960.

The "pound-force" or "lbf" is a measure of force, though. But that's not the pound.

If you want the pound-force in Frink, use lbf or pound force (with no hyphen, which would be indistinguishable from subtraction.) The unit force is a synonym for the unit gravity, which is the standard acceleration of gravity, defined to be exactly 9.80665 m/s2. The "pound-force" is defined as the mass of a pound multiplied by the standard accleration of gravity as defined above.

More details from the (U.S.) National Institute of Standards and Technology:

Appendix B9 of the NIST Guide to SI Units. Please note that the pound is only listed in the mass section and not in the force section. This is from NIST Special Publication 811 which is considered authoritative.
NIST Handbook 133, Appendix E. (This document and its predecessor, NIST Handbook 44, use italics or underlining to show the units that are defined in terms of the survey foot. (I'm glad to see that the 2007 version of this publication apparently contains fixes for most of the several errors that I reported and they acknowledged but sat on for 3 years since first reporting!)
Official definitions from other countries:

From the United Kingdom's National Weights and Measures Laboratory (NWML), the definition of pound as a mass. Also see their FAQ.
U.K. Weights and Measures Act part I, section 1.1, defines the pound as a mass of exactly 0.45359237 kilogram.
Canada's Weights and Measures Act (this is rather fuzzy-headed; it defines the pound as exactly "45 359 237/100 000 000 kilogram" (a mass) but does so under the heading "Measurement of Mass or Weight".) It correctly defines the kilogram as a measure of mass earlier. If the pound is defined as a multiple of the kilogram, and the kilogram is mass, then the pound must be a mass also. This legislation should be amended to remove the misleading heading.
Highly-regarded reference books like the CRC Handbook of Chemistry and Physics, 83rd Edition (2002-2003) has the definition given above in terms of the kilogram. The pound is, again, a mass and nothing else. (This is also true in the 1960 edition, but the 1960 edition has some non-self-consistent uses of "foot pound" as a unit of energy which has been corrected in later versions, which cite only "foot pound-force" as a measure of energy. Thanks to Bob Williams for the historical research.)

I was surprised too when I first started researching the pound. I had been told it was a unit of force by one engineering professor, and I believed it. It turns out he was wrong, I was wrong, and I realized I had better unlearn my mistakes and start using the right terminology before I made a big, costly blunder. If you don't believe it, please do your own research and it might help change your mind. You don't have to believe me, but I think you should believe your own country's standards bodies (and probably comply with your country's legislative definitions if you don't want to breach contracts!) After all, if you don't use the units properly when they're unambigously defined by both standards bodies and law, then you're the one with the liability.

If you can find any evidence that a standards body in the United States or any other country has ever defined pound as anything but mass (well, at least in the past century,) please send it to Alan Eliasen. Please, authoritative references only--not some individual's webpage or old confused textbook.

Yes, I have this discussion over and over again.

"I know that most men, including those at ease with problems of the greatest complexity, can seldom accept even the simplest and most obvious truth if it be such as would oblige them to admit the falsity of conclusions which they have delighted in explaining to colleagues, which they have proudly taught to others, and which they have woven, thread by thread, into the fabric of their lives." --Leo Tolstoy

If you want the British currency, use GBP or Britain_Pound, or, for the historical buying power of the pound in, say, the year 1752, try pound_1752.