'arch-traitor' is a compound formed in English from the prefix 'arch-' and the noun 'traitor'; 'arch-' ultimately comes from Greek (ἀρχ-) meaning 'chief, principal', while 'traitor' comes from Old French 'traitour' and ultimately Latin 'traditor', meaning 'one who hands over' or 'betrays'.

'traitor' developed from Latin 'traditor' → Old French 'traitour' → Middle English 'traitour' → modern English 'traitor'. The combining prefix 'arch-' (from Greek via Latin/Old French in compounds) was attached to denote 'chief' or 'extreme' to form compounds like 'arch-enemy' and later 'arch-traitor'.

The parts kept their original senses: 'arch-' as 'chief/principal' and 'traitor' as 'one who betrays', so 'arch-traitor' has consistently meant 'chief betrayer' or 'principal traitor' in modern usage.