"informer," 1785, probably from underworld slang meaning "the nose" (1700), which apparently developed from an earlier meaning "fillip on the nose" (1670s). Snitcher in same sense is from 1827.
An informer; rat, stool pigeon: Maybe some of my old snitches have run across something new (1785+)
verb[noun and first verb senses probably fr underworld slang snitch, ''nose'']