a) The act of completely destroying an opponent to the point of embarrassment.
b) To beat severely about the head and shoulders.
(see also: pwn)
"Man, we were playing team slayer on Halo 2 last night, and got totally mollywhomped 50-7."
"A young punk got completely mollywhomped last night leaving 2 black eyes and severe bruises that have him in critical condition."
The act of running into a huge crowd and hitting your enemy in the back of the head as hard as you can with the intention of catching them by surprise and knocking them out and the hittee wont know who the hitter was.
-I heard that john slept with your gurl.
--Oh man, john better stay out of crowds or i'm gunna mollywhomp his ass.