sneak
n. countablen. a person who acts in a secret or dishonest way, often to get someone else in trouble. It is a casual word for someone who tells on others or hides their actions.
n. a person who behaves in a furtive, underhanded, or deceitful manner; often used to describe someone who informs on others. Informal in register.
Don't be a sneak and tell the teacher.
The office sneak was caught looking through the manager's private files during the lunch break.
While he maintained a facade of loyalty, his colleagues eventually realized he was a sneak who reported their every minor infraction to the board.
Possibly from Middle English sniken (“to creep, crawl”), from Old English snīcan (“to creep, crawl”), from Proto-West Germanic snīkan, from Proto-Germanic snīkaną (“to creep, crawl”), which is related to the root of snake. Compare Danish snige (“to sneak”), Swedish snika (“to sneak, hanker after”), Icelandic sníkja (“to sneak, hanker after”). Possibly related to snitch.