Adjective. To Walrus a person, they must be laying down either on their back or stomach. The action of "Walrusing" is to leap onto the victim, landing on your stomach. This is similar to a bellyflop... except onto a person.
It is named "walrus" because Walruses lay on their stomachs pretty much all the time.
"Me and Jen were laying on her bed when suddenly she walrused me! She laughed because I made a weird noise due to the lack of breathing that I could do."