Sancho-"I'll be fuckin good to her mate"
Alf -"You better be fuckin good to her mate, cause I'll fuckin skulldrag you back to Mexico by your fuckin stupid mullet and fill your ass with nachos"
The term actually refers to tying someone's feet to the back of a horse or vehicle and dragging them. Hence their skull would drag on the ground. But in practice it means to jack someone up, either through yelling at them, "smoking" them through push-ups, assigning extra duty or generally being a prick and giving them a difficult time.
Soldier 2: Pipe down man, if the platoon sergeant here's you say that he'll skull drag your ass.