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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.