@Setanta,
That raises the question of why so many words related to the human body and it's functions have been chosen for use as pejoratives.
I'm guessing but it appears the function of a pejorative is to instill shame.
Which says something about the role that shame, in our own bodies, plays in society.
On that basis, the OP's question, in terms of swear used as pejorative, could be answered as "to cause shame".
Along this line, a few years ago I found a bumper sticker, which I gave to a friend, "You say I'm a prick like that's a bad thing"