means To snitch is to inform on someone, reporting their secrets or wrongdoing to an authority — or, as a noun, the person who does it.

from "Snitch" surfaces in 18th-century thieves' cant, where it first meant a fillip on the nose, then the nose itself — and a "snitch" became someone who, in the slang logic of the underworld, stuck their nose into others' business or turned their nose toward the law. The leap from "nose" to "informer" runs through the same instinct that gave us "to nose out" a secret. The exact bridge is murky, as criminal slang rarely leaves a paper trail, but the nose-to-narc journey is well attested.