Bun

Bun

n.
  1. 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.
  2. a type of coiffure in which the hair is gathered into a coil or knot at the top of the head.
  3. the buttocks. [slang]

BUN

n.
  1. 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.)