1. Slang for a pathetic hack.
2. Slang shameless hack.
3. One who appears to trumpet his or her lack of originality.
Terms are derived from the characteristics of popular television program The Tonight Show's host, Jay Leno. It refers to him bribing away talent from their original sources instead of cultivating his own (see John Menendez, formerly of The Howard Stern Show, and That Glasses Guy, formerly from The Daily Show With John Stewart) as well as blatantly stealing comedy bits from other comedy shows (see Celebrity Jeopardy skits, originally performed on the program Saturday Night Live).
"Billy overheard the premise for my Science Fair project and he totally Jay Leno-ed me."