The act of aiming only to wreck/spinning out the first place person in a racing video game; much like the spiked shell from Mario Kart. Often frowned upon and seen as immature.
John: Me and Mike spent all last night spiked shelling in Grid. We almost took out the whole field, one by one.
Greg: You're a douche. Grow up.
A reference to young men who grow scruffy beards, wear their hair in a greasy and disheveled fashion and wear surplus military field jackets in an apparent attempt at duplicating the stereotypical media image of a traumatized war veteran.
That young twenty-something with the darting eyes and the spastic movements whom I saw sitting in front of the English department seems to be affecting quite an elaborate shell sham.