late 14c., prollen, "move about in search of something," of unknown origin, with no known cognates. Spelling with -w- is from 1500s (cf. bowl), but pronounced "prôll" till late 18c. Meaning "go stealthily in search of prey" is first recorded 1580s. Related: Prowled; prowling. The noun, in on the prowl, is attested from 1803.
Actively looking for something, as in Their underpaid computer programmers are always on the prowl for better jobs. This idiom transfers an animal's search for prey to human pursuits that are usually less bloodthirsty. [c. 1800]