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1.
Another word for idiot or a weirdo or someone that has done something silly.
'Your such an nimbrain sometimes!'
by KC&GD January 27, 2012
 
2.
A polite way of calling someone an Idiot.
Girl: I'm so much cooler than you!

Guy: Oh, you really are a Nimbrain!
by thecoolestguyintheworld February 16, 2012
 
3.
Someone who says/does something or is stupid.

A polite was of saying Retard or Dickhead etc.
Kathryn: I'm so much cooler than you!

George: Oh dear, you really are a nimbrain!
by supercoolguy February 11, 2012