Usually used to mean acting and dressing flamboyantly for women, but the word's roots actually lay in the competitive eating circuit.
To peacock is to thrust one's gut outwards in a flamboyant fashion. The peacocker will parade about the room with his gut out for the world to see. It's a sign of pomposity often seen after a hard fought victory. More and more though, arrogant, up and coming eaters will peacock in public settings or at a non-competitive eating function. This is offensive and occasionally leads to fights.
While there's nothing wrong with a little peacocking here and there, one should not abuse their gut and peacock too frequently. One should respect their gut and the guts of others by keeping their own gut to themselves. Only peacock when you have earned the right!
Slugger peacocked all day, everyday for a straight week, needless to say, Acun and Big Kahuna were outraged and challenged him the next day at the local Golden Corral.