1. Somebody really ripped and muscular. Usu. male.
2. Used ironically to describe somebody extremely overweight. Popularized by an early episode of South Park in which Cartman drinks weight gain formula and gets really huge.
3. A cake made of meat. Almost always beef.
When a bodybuilder has bulked up so much, that their armswill no longer lay at their sides. Instead, the arms dangle out at an angle, and the individual takes on the characteristics of a strutting chicken. This is due to muscles on the arms resting against more muscle on the back and sides.