It's from a film of the same name, itself referring to the older expression 'In Like Flynn', Errol Flynn who had a reputation as a womaniser. http://en.wikipedia.org/wiki/In_Like_Flint

the saying i know is in like flyn, and it refered to eyrol flyn.

In Like Flint is a spoof spy film (series)featuring James Coburn as the 007 type agent with more gadgets than the man himself and more testerone and females and totally tongue in cheek (or elsewhere) See www.imdb.com for info

Okay, this touches on all your input: The line re: "in like Flint" was first a line used in print ads for a 1940's product, "Flint" lighter fluid starters (a flint is the metal or steel piece the lighter strikes against to produce a spark.) (Years later, the movie made a pun on the title.) And the Flint ads were taking advantage of the familiarity of the phrase from the late 30's: "in like Flynn" -- which was a shorthand term used when pitching movie story ideas, referring to a fast and sweeping entrance, as Errol Flynn was famous for as he swung into scenes on a rope, vine, curtain, sail, etc. A writer would pitch: "so then our robber comes in like Flynn and sweeps the clerk off her feet..." -- it was shorthand to the producers who all knew what he meant. HOWEVER -- when Errol's scandal with the teens broke, it took on a rude meaning with the tabloids of the time, and "in like Flynn" was slang for "got an easy lay quickly." RECAP: "In like FLYNN" was first shorthand re: Flynn's movie entrances; then a sexual pun; then an ad or two for the Flint company, then a movie title punning the latter two. Phrew!

just had a look at a site which appears to contradict the connection with Errol (in like Flynn)by some years www.worldwidewords.org

I am answering my own question because apparently we were all wrong except for "oldmanuk." It's his second post that is correct. Thanks! Reference books almost universally assert that this set phrase, an American expression meaning to be successful emphatically or quickly, especially in regard to sexual seduction, refers to the Australian-born actor Errol Flynn. His drinking, drug-taking and sexual exploits were renowned, even for Hollywood, but the phrase is said to have been coined following his acquittal in February 1943 for the statutory rape of a teenage girl. This seems to be supported by the date of the first example recorded, in American Speech in December 1946, which cited a 1945 use in the sense of something being done easily. The trouble with this explanation is that examples of obviously related expressions have now turned up from dates before Flynn’s trial. 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”. It’s suggested by some writers that the phrase really originated with another Flynn, Edward J Flynn — “Boss” Flynn — a campaign manager for the Democratic party during FDR’s presidency. Flynn’s machine in the South Bronx in New York was so successful at winning elections that his candidates seemed to get into office automatically. The existence of the examples found by Mr Popik certainly suggest the expression was at first unconnected with Errol Flynn, but that it shifted its association when he became such a notorious figure. Since then, it has altered again, because in 1967 a film, In Like Flint, a spy spoof starring James Coburn, took its title by wordplay from the older expression, and in turn caused many people to think that the phrase was really in like Flint. http://tinyurl.com/a2zr5