late 14c., poisen, "to have (a specified) weight," a sense now obsolete, from Old French poiser, stressed form of 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 hang, cause to hang; weigh; pay" (from PIE root *(s)pen- "to draw, stretch, spin").

The meaning "to weigh, ascertain by weighing or balancing is from 1590s, hence the meaning "to hold or place in equilibrium or balance," from 1630s (compare equipoise). The intransitive sense of "be balanced or suspended," figuratively "to hang in suspense" is by 1847; the passive sense of "to be ready" (for or to do something) is by 1932. Related: Poised; poising. In 15c. a poiser was an official who weighed goods. The secondary sense of "to ponder, consider" in Latin pensare yielded pensive; that sense was occasional, but rare in Middle English poise.

Proto-Indo-European root meaning "to draw, stretch, spin."

It forms all or part of: append; appendix; avoirdupois; compendium; compensate; compensation; counterpoise; depend; dispense; equipoise; expend; expense; expensive; hydroponics; impend; painter (n.2) "rope or chain that holds an anchor to a ship's side;" pansy; penchant; pend; pendant; pendentive; pending; pendular; pendulous; pendulum; pension; pensive; penthouse; perpendicular; peso; poise; ponder; ponderous; pound (n.1) "measure of weight;" prepend; prepense; preponderate; propensity; recompense; span (n.1) "distance between two objects;" span (n.2) "two animals driven together;" spangle; spanner; spend; spider; spin; spindle; spinner; spinster; stipend; suspend; suspension.

It is the hypothetical source of/evidence for its existence is provided by: Latin pendere "to hang, to cause to hang," pondus "weight" (perhaps the notion is the weight of a thing measured by how much it stretches a cord), pensare "to weigh, consider;" Greek ponos "toil," ponein "to toil;" Lithuanian spendžiu, spęsti "lay a snare;" Old Church Slavonic peti "stretch, strain," pato "fetter," pina "I span;" Old English spinnan "to spin," spannan "to join, fasten; stretch, span;" Armenian henum "I weave;" Greek patos "garment," literally "that which is spun;" Lithuanian pinu "I plait, braid," spandau "I spin;" Middle Welsh cy-ffiniden "spider;" Old English spinnan "draw out and twist fibers into thread," spiðra "spider," literally "spinner."