An informer is one who hopes for a reward in return for information.
An incredible amount of slang has been invented to say “informer” —snitch, fink, nark, stoolie, stool pigeon, blabbermouth, squeaker, squeak, and many more.
Let’s examine the British word grass (sometimes grasser). Grass/grasser used as a noun, means, “one who informs, tells tales, squeals.” Grass was first used this way in 1932.
Grass also appears as a verb (to inform). It’s thought to be rhyming slang, short for “grasshopper” which equates with “shopper.” Sometimes we hear “grass in the park,” also rhyming slang, this time for “nark” or “narc.”
Nark (mid-19th century) is older than grass. Always spelled with a “k” back then, it meant “police informer.” In the 1950s, we began to see narc spelled with a “c.” It then generally referred to a narcotics agent. However, since both words sound the same, narc also began to mean “informer.” To narc one over is to expose a drug dealer. This phrase entered U.S. English in the 1950s.
To squeal and squealer are mid-19th century. In the 1960s, a U.S. squeal rule or squeal law was enacted requiring doctors to notify parents when an underage girl requests a contraceptive prescription.
Squeaker is old, having come into English in 1690. Squeak is from the same time. Both words generally refer to someone who’s been arrested and turns informer to save himself.
The oldest speech-related slang for informer are blab and blabber which entered vocabulary in the early 1600s. They’ve always referred to “tattletales.”
The earlier non-slang blabble (chatterer) is found in Chaucer (c. 1370). Earlier still is the verb blabber, which is used in Piers Ploughman (1362), and the related noun blabberer used by Wyclif (1830-84).
While most expressions about informing are viewed in a negative way, we think differently about the noun whistleblower (1970). Whistleblower (sometimes hyphenated) evolved from the phrase, “to blow the whistle on,” first used in 1934. To blow the whistle on, as well as meaning, “to inform against,” has the second meaning of “to bring to an end.”
Most whistleblowers are seen as people who expose wrongdoing and criminal activity in the workplace. This is curious, since to blow the whistle on originated in the underworld.
Also from the underworld is fink, used for "informer” (1920s). Back in the 1890s, fink meant “strike-breaker” (scab).
To finger or, to put the finger on was originally underworld slang, as well.
Snitch has meant “informer” since 1785. Originally found as “turn snitch,” snitch has also been slang for “nose” since the mid-17th century. Turn snitch on, probably initially meant, “to stick one’s nose in.”
We also have the mid-1800s’ chirp, to rat, to sing, all from the U.S., as is the 1930s-era word stool pigeon.
Other countries have words for informers, too. In French, there are, le delateur and le dénonciateur. French slang for “a snitch” is le mouchard.
Since 1916, Australian slang for “informer” is shelf, and to betray someone to police is to dob in (1955).