"In like Flynn" is a slang phrase meaning "having achieved a goal or gained access as desired". In addition to its general use, the phrase is sometimes used to describe success in sexual seduction, and its folk etymology often asserts the phrase has sexual origins.
The term is often believed to refer to movie star Errol Flynn. Flynn had a reputation for womanizing, consumption of alcohol and brawling. His freewheeling, hedonistic lifestyle caught up with him in November 1942 when two under-age girls, Betty Hansen and Peggy Satterlee, accused him of statutory rape. A group was organized to support Flynn, named the American Boys' Club for the Defense of Errol Flynn (ABCDEF); its members included William F. Buckley, Jr. The trial took place in January and February 1943, and Flynn was cleared of the charges. According to etymologist Michael Quinion, the incident served to increase Flynn's reputation as a ladies' man, which led to the popular phrase "in like Flynn". Columnist Cecil Adams also examined the term's origins and its relationship to Flynn. Many early sources, attesting the phrase, say it emerged as war slang during World War II.
In addition to the Errol Flynn origin theory, etymologist Eric Partridge presents evidence that it refers to Edward J. Flynn, a New York City political boss who became a campaign manager for the Democratic party during Franklin Delano Roosevelt's presidency. Boss Flynn's "Democratic Party machine exercised absolute political control over the Bronx.... The candidates he backed were almost automatically 'in'."
Barry Popik of the American Dialect Society found an example from 1940, as well as this from the sports section of the San Francisco Examiner of 8 February 1942: "Answer these questions correctly and your name is Flynn, meaning you're in, provided you have two left feet and the written consent of your parents". To judge from a newspaper reference he turned up from early 1943, the phrase could by then also be shortened to I'm Flynn, meaning "I'm in".
Quinion also notes that the title of the film In Like Flint (1967) is a play on the term, and that has led to a malapropism where some speakers believe that is the original phrase.