The most heinous or infamous traitor; someone who betrays in the most serious way possible.
From 'arch-' (chief) plus 'traitor' (from Old French 'traître', derived from Latin 'traditor', one who hands over). This intensifying compound reflects medieval legal and moral language about betrayal.
The word 'archtraitor' appears frequently in historical accounts of political upheaval—think Benedict Arnold or Judas Iscariot. It shows how languages need superlatives to express the worst form of human betrayal!
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