Most likely from the French 'luncheon', lunching burst into the American lexicon circa 1992 in the Washington DCMetropolitan Area.
Intransitive verb: to laugh hysterically/uncontrollably
Transitive verb: causing above in another person
Intransitive: "I just saw a fat man driving a tiny car and I started lunching my butt off."
Transitive: "This guy... this guy right here... is lunching me out!"