[fundamental unit of weight] Old English pund "pound" (in weight or money), also "pint," from Proto-Germanic *punda- "pound" as a measure of weight (source of Gothic pund, Old High German phunt, German Pfund, Middle Dutch pont, Old Frisian and Old Norse pund), an early borrowing from Latin pondo "pound," originally in libra pondo "a pound by weight," from pondo (adv.) "by weight," ablative of pondus "weight," from stem of pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin"). Perhaps the notion is the weight of a thing measured by how much it stretches a cord.
Meaning "unit of money" was in Old English, originally "a (Tower) pound of silver."
In the Middle Ages it was reckoned variously: the Tower pound (12 ounces), the merchant's pound (15), the avoirdupois (16), the Troy (12); the 16-ounce pound was established before late 14c. Pound cake (1747) is so called because it has a pound, more or less, of each ingredient. Pound of flesh is from "Merchant of Venice" IV.i. The abbreviations lb., £ are from libra "pound," and reflect the medieval custom of keeping accounts in Latin (see Libra).
A peny yn seson spent wille safe a pounde. [Paston Letters, 1457]