This can be a verb, meaning to inform or to tell tales, or it can be the person that does so. It dates in this sense from the late 18th century and derives from earlier slang when snitch was a common word for nose. The word nose has long been associated with nosing into other people’s business and gave rise to the word nosey, which dates from the 1700s. In fact, nose has been slang for an informer since the late 1700s. Therefore, it was but a short step, etymologically speaking, for the word snitch to mean nose and then someone who informs. People who snitch or inform use their nose in a figurative sense to sniff around and we still use the word sniff in this context. For example, newspaper reporters often sniff around for a story. See also grass and tittle-tattle.