Definition - one who snitches; a tattletale
We’re not sure where the word snitch comes from, although we do know that snitches have been snitching for a very long time. The noun form has been in use since the 17th century; originally with meanings such as “nose,” or “ a blow on the nose,” and in the 18th century taking on its “tattletale” sense. Snitch as a verb followed soon after. The phrase snitches get stitches appears to be considerably newer, as we only have evidence of its use from the past few decades.
He said correction officers involved in clashes often strike agreements to “squash the incident,” while prisoners who consider reporting battles with other inmates are dissuaded by the jailhouse maxim that “snitches get stitches.”
— Susan Milligan, The New York Daily News, 21 Oct. 1986