Somebody who is deceptive, ignorant or otherwise unpleasantly cunning. Not a friendly, jovial term, a Sackhead is very much similar to a dickhead, in which said person is claimed to have a penis, or in this case a (Nut)'Sack' upon their head.
When your girl pulls a nylon stocking down over her head and sucks your dick. The feel of her nylon lips and wet silky tongue against your shaft is such a rush. It also catches most of the spooge so she can choose to eat it or wear it.