When a simple 'fuck you' will not suffice. Used in extreme cases. Note: Use with care. An over-use of this word causes a severe decrease in its value and over all special-ness.
Tina's fiance just told her that he is leaving her for her maid of honor, Janis, will be going on their honey moon with Janis, will NOT provide child-support for his and Tina's unborn son, and that Tina looks fat in her wedding dress.
Grandpa looks down at his grandson after he tells a hella of a lie and tells him grandson you can "Fuck an old fucker but you cant shit an old shitter", go tell that bull shit story to some younger generation because I'm not buying it.