Character conceived of by filmmaker Kevin Smith to be the drug-dealing comic foil for his alter-ego, Silent Bob. Played by actor Jason Mewes.
Jay appeared first in Clerks, then Mallrats, Chasing Amy, Dogma, and Jay and Silent Bob Strike Back. He and Silent Bob also make a cameo appearance in Scream 3 and lend their voices to Clerks: The Animated Series.
According to Kevin Smith, the Jay character was basically a synthesis of a younger version of Mewes himself. Jason, who was originally straightedge and then jumped head-first into the world of drugs, alcohol, and sex, was prone to being fairly silly (screaming things like "I'll fuck anything that moves!") and lent himself quite well to be the talkative, slender accompaniment of Silent Bob.
A few notes on Jayspeak:
Throughout all of the so-called "New Jersey Trilogy" films, Jay had a particular way of speaking, utilizing peculiar words and phrases such as "snoogans" and "snoochie boochies." Smith explained that this was again an example of something he adopted from Mewes' real life.
Apparently, Jason used to be fond of using the made-up word "neh" to mean "I'm just kidding, so please don't kick my ass." For example:
"Your momma wears army boots, neh."
Smith said that as the years progressed, the word/phrase evolved. "Neh" became "nooch," (e.g. "Your momma wears army boots, nooch."). "Nooch" became "snooch," "snooch" became "snoochie boochies," and "snoochie boochies" became "snooch to the nooch."
Eventually, said
Smith, the word reached its
zenith, with Mewes walking around saying stuff like:
"Your momma wears army boots, snikadikadointch."
And finally, the word shrank back down (presumably for ease of use) to simply "snoogans," which is the contemporary term of preference. Despite all its iterations, the word still means "I'm kidding" and "don't kick my ass," according to Mewes.
Sources:
1. imdb.com
2. An Evening with Kevin Smith, Sony Pictures, 2002. (ISBN: 0-7678-9367-0)