A spin-off of Al Gores manbearpig. Used to describe someone using the bodybuilding supplement Russian Bear 5000 which is a high protein, high calorie weight gainer that among other things contains Sickadeer antler powder. Therefore becoming 1/2 man 1/2 bear 1/2 deer. Manbeardeer!
Guy 1: Who's that guy over there bench pressing 800 pounds?
Guy 2: That guys always in here throwin weight around. Thats manbeardeer!