From The Collaborative International Dictionary of English v.0.48:
Bun \Bun\, Bunn \Bunn\, n. [Scot. bun, bunn, OE. bunne, bonne;
fr. Celtic; cf. Ir. bunna, Gael. bonnach, or OF. bugne tumor,
Prov. F. bugne a kind of pancake; akin to OHG. bungo bulb,
MHG. bunge, Prov. E. bung heap, cluster, bunny a small
swelling.]
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.
[1913 Webster +PJC]
2. a type of coiffure in which the hair is gathered into a
coil or knot at the top of the head.
[PJC]
3. pl. the buttocks. [slang]
[PJC]
From The Collaborative International Dictionary of English v.0.48:
BUN \BUN\ n. [acronym] (Med.)
same as
blood urea nitrogen; the concentration of nitrogen in blood
present in the form of urea; -- used as a measure of
kidney function.
Note: Blood usually contains 10 to 15 mg of nitrogen per 100
ml in the form of urea. --Stedman.
[PJC]