1. Quite literally, to fuck in the skull. This usually means inserting one's penis into any orifice
in the head (excluding the mouth), such as the nostrils, ears or a hole bashed into the back of the head with a pair of pliers. As this is not only rather painful but quite
impossible in most cases, skullfucking is not a common practice in real life (unless you're talking about necrophiliacs that is). It is, however, tolerably prevalent in the world of hentai art, resulting in pictures that are more horrific than sexually arousing. My advice: don't seek out any of these pictures unless you have an
uncontrollable urge to lose your lunch all over the keyboard. See also skullfuck.
2. See mindfuck.
3. Can also be used as a kind of insult.
