Bun

any of a variety of slightly sweetened or plain raised cakes or bisquits, often having a glazing of sugar and milk on the top crust; as, a hot cross bun.

a type of coiffure in which the hair is gathered into a coil or knot at the top of the head.

the buttocks.[slang]

same as blood urea nitrogen; the concentration of nitrogen in blood present in the form of urea; -- used as a measure of kidney function.(Med.)