1660s, "pertaining to conjecture," from Greek stokhastikos "able to guess, conjecturing," from stokhazesthai "to guess, aim at, conjecture," from stokhos "a guess, aim, fixed target, erected pillar for archers to shoot at," perhaps from PIE *stogh-, variant of root *stegh- "to stick, prick, sting." The sense of "randomly determined" is from 1934, from German stochastik (1917).
