Where did the saying "in like flin" come from?

The earliest known use of "in like Flynn" in print is in the December 1946 issue of American Speech. Penn State prof Ed Miller reported that students of his who had served in the army air force during World War II used the expression to mean, "'Everything is OK.' In other words, the pilot is having no more trouble than Errol Flynn has in his cinematic feats."

From this we learn several things: (1) The expression was of recent origin. Had it been widely used in the 1930s Miller would not have included it in a list of World War II slang. (2) The term was generally understood to refer to Errol Flynn. (3) It didn't necessarily refer to Flynn's success with women. (4) Then again, maybe Miller's students just didn't feel like proclaiming otherwise in the middle of class.

No question, a lot of people think the phrase means in like Flynn's you know what, and with good reason. Flynn, a popular film star in the 1930s, became notorious in November 1942 when he was charged with two counts of statutory rape. Though acquitted he was the butt of jokes ever after.

One film bio none too subtly comments, "Warner Brothers . . . found [Flynn's] popularity not only had held but had a new spurt of interest. A new phrase was added to the English language: `In like Flynn'" (Tony Thomas et al, The Films of Errol Flynn, 1969).

Another says the posttrial Flynn became a "wild man of the mattress. The slogan 'In like Flynn' rose like smoke from the trial and ran laughingly around the globe" (Earl Conrad, Errol Flynn: A Memoir, 1978).

An Australian playwright (Flynn was born in Australia) even claimed the expression "in like Errol" was current in his country for a time (Alexander Buzo, Rooted, 1973).

But still. These guys were writing after the sexual revolution of the 1960s. In like Flynn's schwanz? In the 1940s?

An alternative interpretation comes from A Dictionary of Catch Phrases (Eric Partridge, 1986). Edward J. Flynn (1892-1953) was a New York City political boss who became a campaign manager for the Democratic party during FDR's presidency. Boss Flynn's "Democratic Party machine exercised absolute political control over the Bronx.... The candidates he backed were almost automatically 'in,' and he himself permanently so," Partridge comments.

Now we have the beginnings of a theory. "In like Flynn" starts as rhyming slang in New York, helped along by the prominence of Boss Flynn. NYC draftees spread it among the troops nationwide with the start of World War II. The phrase gets a boost when the well-publicized travails of Errol Flynn in 1942 give it a double meaning. But its innocent origin allows Cecil's mom to use it without being scandalized.

Just one thing. We have no evidence that "in like Flynn" was used anywhere prior to November 1942.
http://www.straightdope.com/classics/a5_156.html

It's actually "In like Flint", a movie from the 60's about a James Bond-like character. There is also the "In like Flin" saying that a lot of people attribute to the sexual conquests of actor Errol Flin. I think most people today mean the former.