late 14c., prollen, "rove or wander in a stealthy manner, move about in search of something," a word of unknown origin, with no known cognates. Spelling with -w- is from 1500s (compare bowls), but the word was pronounced "prôll" till late 18c. Transitive meaning "go stealthily over, as one in search of prey or plunder" is recorded by 1580s. Related: Prowled; prowling. The noun, in on the prowl, is attested from 1803.