PB&JJ is an abbreviation for a penis butter and jelly job. This is the act of sticking your penis into a jar of jelly and then receiving a blow job. Try warming the jelly first, but be warned of the temptation of turning the PB&JJ into a simple PB&J.
Ususally used as an inside joke between family guy fans. Although they may think that it is an inside joke, it isn't because almost everybody knows it.