unstack (v.)

prefix of reversal, deprivation, or removal (as in unhand, undo, unbutton), Old English on-, un-, from Proto-Germanic *andi- (source also of Old Saxon ant-, Old Norse and-, Dutch ont-, Old High German ant-, German ent-, Gothic and- "against"), from PIE *anti "facing opposite, near, in front of, before, against" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before").

More or less confused with un- (1) through similarity in the notions of "negation" and "reversal;" an adjective such as unlocked might represent "not locked" (un- (1)) or the past tense of unlock (un- (2)).

early 14c., "to pile up (grain) into a stack," from stack (n.). Meaning "arrange (a deck of cards) unfairly" (in stack the deck) is first recorded 1825. Stack up "compare against" is 1903, from notion of piles of poker chips (1896). Of aircraft waiting to land, from 1941. Related: Stacked; Stacking.