"butt" is short form for the word "buttock or buttocks". A buttock (singular) is the fleshy part that we sit on. Buttocks would then mean both round fleshy parts that forms our "back end, backside, behind, bottom, bum, butt, derriere, fanny, gluteus maximus, haunches, hindquarters, posterior, rear, rump, seat, cheeks, and ass". So, "Sit your butt down" means "sit down", however, it is a disrespectful expression, usually said in anger, and should never be used in formal situations.

I think it usually just means "sit down," but either more emphatically or humorously. At least my own own experience and usage!

Sit your butt down. Person A would say this to Person B if Person B was walking around a lot (pacing) back and forth and Person A found this to be very annoying. (for example). Person B might be worried about something or angry or upset. Person A is tired of watching, looking at or listening to Person B. So Person A says... "Would you please sit your butt down!" <<<< This means ... "Would you please stop (doing what you are doing that is annoying/bothering me) and sit down in a chair please?!" It is more of a strong request or command from Person A to Person B. So it means that Person A found what Person B was doing very irritiating. I hope this is clear. Sometimes Person A might say "damn" < a swear word - just before the word "butt" just to make the feeling more strongly expressed to Person B. Sorry to teach an obscenity but I don't think it is very offensive these days to say "damn".

What about "Quit smoking!" ?? ;-)

be still,,,,