A modifier used to intensify a command. Slightly more powerful than "your ass." Appending "bitch" onto the end of the sentence further strengthens the emphasis as well as the condescendingpower.
From most polite to most offensive/aggressive:
"Please sit down if you would be so kind."
"Sit down, please."
"Sit down."
"Sit your ass down!"
"Sit the fuck down!"
"Sit the fuck down, bitch!"
What the fuck is that?
Where the fuck did Jim go?
Why the fuck did that happen?
How the fuck you gonna do that?
When the fuck is the bus gonna get here?