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A Warrior. In Irish myth he was the dude who ran away with Fionn McCughall's wife. He was also good at climbing. Nowadays Diarmuids are generally of a good stock. Although often confused with the inferior anglo bastardisations of the proper name such as Dermot, Dermott and Diarmaid. Due to their relative rarity they tend to be unique, always quick witted, charming and easy going and It's probably worth noting that the name translates from the Gaelic as 'Without Fear or Envy'.

Noun: A sly motherfucker who'll wind you up and win your respect in the process with his acid tongue.
You better get offa my mother Diarmuid you crazy, charming bastard.
by reynard the fox March 11, 2011