"Just joshing", thought I. "I'll bet that phrase is a reference to Josh Billings.!" So today I had another idle minute and searched briefly. Here's what I found:
So there it is. Maybe. Maybe not. Possibly. Maybe.
Posted by bob on October 03, 2000
In Reply to: Re: Josh posted by ESC on October 02, 2000
: : Where did the phrase 'you're joshin' me', come from? If you know, please e-mail me...
: "JOSH - The best guess is that the Americanism 'josh,' for 'to kid' or 'fool around,' is a merging of 'joke' and 'bosh.' The pseudonym of an American writer may have something to do with the word, though. Henry Wheeler Shaw (1818-85) wrote his deliberately misspelled crackerbox philosophy under the pen name Josh Billings. Employing dialect, ridiculous spellings, deformed grammar, monstrous logic, puns, malapropisms, and anticlimax, he became one of the most popular literary comedians of his time. The expression 'to josh' was used about 18 years before Josh Billings began writing in 1863, but his salty aphorisms probably strengthened its meaning and gave the term wider currency." From "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Fact on File, New York, 1997)
The OED concurs, suggesting that he chose the pseudonym from the existing slang word and his popularity promoted its usage. Anybody have a clue why so many 19th century writers wrote under pen names? There are lots of 18th and 20th c. pen names, sure, but it really seems to have flourished in the 19th. I haven't a clue.
The site I found this on, by the way, is very useful if you're looking for the origins of phrases.