1660s, "pertaining to conjecture," from Greek stokhastikos "able to guess, conjecturing," from stokhazesthai "to guess, aim at, conjecture," from stokhos "a guess, aim, target, mark," literally "pointed stick set up for archers to shoot at," from PIE *stogh-, variant of root *stegh- "to stick, prick; pointed" (see sting, v.). The sense of "randomly determined" is from 1934, from German stochastik (1917).