Smack your hands against your body repeatedly, mainly the torso to chest area, and continue to smack hands on body and say "hambooooniiinngggg" in Rigby's voice from Regular Show.
Rigby: HAMBONINGGGG
Mordecai: No...
Rigby: HAMBONING WILL SAVE YOUR LIFE ONE DAY!!!! It'll be like "what! you tried to mug me?"
*proceeds to hambone for 20 seconds*
Mordecai: NO! We're not doing that, okay!? Okay!?
Rigby: Fine...
Hambeast is one of those words which may have originated with Something Awful. It refers to people who are porcine, that is, piggishly fat, with arms and legs like hams.
"Look at this picture and tell me Fud isn't a hambeast. How much does he eat in a day?!"