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!"
A girl is eating her lunch (or any meal really) and she is shitting it out while you eat the shit. If you please, you may also regurgitate the shit into her mouth and she eats it, shits it out, and the process continues.