early 15c., "weight, quality of being heavy," later "significance, importance" (mid-15c.), from Old French pois "weight, balance, consideration" (12c., Modern French poids), from Medieval Latin pesum "weight," from Latin pensum "something weighted or weighed," (source of Provençal and Catalan pes, Spanish, Portuguese, Italian peso), noun use of neuter past participle of pendere "to weigh" (see pendant).
The sense of "steadiness, composure" first recorded 1640s, from notion of being equally weighted on either side (1550s). Meaning "balance" is from 1711; meaning "way in which the body is carried" is from 1770.
poise (v.)
late 14c., "to have a certain weight," from stressed form of Old French peser "to weigh, be heavy; weigh down, be a burden; worry, be a concern," from Vulgar Latin *pesare, from Latin pensare "to weigh carefully, weigh out, counter-balance," frequentative of pendere (past participle pensus) "to weigh" (see pendant). For form evolution from Latin to French, see OED. Meaning "to place in equilibrium" is from 1630s (compare equipoise). Passive sense of "to be ready" (to do something) is from 1932. Related: Poised; poising. In 15c. a poiser was an official who weighed goods.