Definition of bun noun from the Oxford Advanced Learner's Dictionary

bun

noun
bʌn
 
; bʌn
 
 
1 [countable] (British English) a small round sweet cakean iced bun see also hot cross bun2 [countable] (British English also bap) a small round flat bread rolla hamburger bun compare roll3 [countable] long hair that has been twisted into a round shape and is worn on top or at the back of the headShe wore her hair in a bun.4 buns [plural] (slang, especially North American English) the two sides of a person's bottom
Idioms

have a bun in the oven

(informal, humorous) to be pregnant