Other forms: snitching, snitches, snitched
To snitch is to report somebody's wrongdoing to those in charge. Snitch on him, nobody likes a snitch, snitching to the teacher.
The preposition is on. It is playground and prison vocabulary, and it always carries disapproval of the person telling rather than of the person told about — which is the whole social point of the word. Tell tale is the British children's equivalent and grass is the criminal one. Note that snitch also means to steal something small, and that as a noun it is an old slang word for the nose.
1verbsnitchTo report somebody's wrongdoing to those in charge.
“Who snitched to the teacher about the broken window?”
2nounreportsSomebody who reports on others.
“The snitch was found out within about a fortnight.”
snitch is an informing.
The word appeared in the eighteenth century as thieves' slang, probably from an older word meaning a snare or a snatch. Its route is untidy, which is usual for words that spent two hundred years being spoken and not written.
Middle English
snacche — a trap, snare
English, today
snitch
Origin unknown. Perhaps an alteration of Middle English snacche (“a trap, snare”), snacchen (“to seize (prey)”). Compare also Middle English snik snak (“a sudden blow, snap”). Alternatively, perhaps from a dialectal variant of a doublet of sneak, from Middle English sniken, from Old English snīcan (“to creep; crawl”). More at sneak, snatch.
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire