the fleshy part of the human body that you sit on
"he deserves a good kick in the butt"
"are you going to sit on your fanny and do nothing?"
The buttocks or more specifically, the anus.
"The geneticist explained that a mutation affecting ARSE could disrupt normal cartilage formation during fetal development."
Abbreviation of arylsulfatase E, an enzyme, deficiencies in which are associated with abnormalities in cartilage and bone development.
To be silly, act stupid or mess around.
"You really shouldn't have been messing around and playing tricks on everyone during the exam."