A more advanced way of describing the actions of an air hump. First, you put your arms out flat infront of you, then you pull back as hard as you can whilst clenching your fists, at the same time as pushing your pelvis forward in a humping motion. Can be adapted in many ways e.g. in badminton, a player hits the shuttlecock with the racket through their legs. This motion looks as if the player is trying to hump the shuttlecock, therefore they have done a Greg McGrath.
Instead of being normal today, Kumar decided he would be a freak & do the Greg McGrath.