grandmother       
noun [ C ] uk/ˈɡræn.mʌð.ər/ /ˈɡræm.mʌð.ər/ us/ˈɡræn.mʌð.ɚ/ (informal grandma); (granny); (gran)
A2 (外)祖母
the mother of a person's father or mother
  Both my grandmothers were from Scotland. 我的奶奶和外婆都来自苏格兰。 
 [ as form of address ] formal or old-fashioned Sit down here, Grandmother, and rest. 坐这里,奶奶,休息一下。 
 - More examples
 - My grandmother is in her eighties.
 - The apple pie was as good as the one my grandmother used to make.
 - My grandmother was booked for speeding last week.
 - He lives with his grandmother.
 - She was brought up by her grandmother.
 
Idiom  
teach your grandmother to suck eggsidiom UK disapproving 班门弄斧
to give advice to someone about a subject that they already know more about than you