Alley rat

a demoralized, corrupt, thieving person.

Though commonly misconceived as the anal canal through which gerbils at times migrate, a la Richard Gere (that region is better termed "gerbils' boulevard of broken dreams"), rats' alley is a horse of another color. Rats' alley is, in point of fact, not only the anal canal, but also the oral, vaginal, optical, nasal, auditory, wound-ary, navel, penile, areolar, carpal, and phallangular canals through which rats at times not only migrate but defecate upon.