aplomb (n.)youdaoicibaDictYouDict
"assurance, confidence," 1828, from French aplomb (16c.), literally "perpendicularity," from phrase à plomb "poised upright, balanced," literally "on the plumb line," from Latin plumbum "(the metal) lead" (see plumb (n.)), of which the weight at the end of the line was made.
poise (v.)youdaoicibaDictYouDict
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.