Fink

Noun

 * 1)  Someone who betrays a trust.

Verb

 * 1)  To betray a trust.

Thesaurus
bear witness against, betray, betrayer, blab, blabber, blabberer, blabbermouth, blackleg, blow the whistle, canary, delator, inform against, inform on, informer, narc, nark, peach, peacher, rat, scab, sell out, snitch, snitch on, snitcher, spy, squeal, squealer, stool, stool pigeon, stoolie, strikebreaker, talebearer, tattle, tattler, tattletale, tell on, telltale, testify against, tipster, turn informer, whistle-blower

Etymology
finco:.

Noun

 * 1) finch