'archthief' originates from Modern English as a compound combining the prefix 'arch-' (from Greek 'arkhos'/'archi-' meaning 'chief, principal') and the word 'thief' (from Old English 'þēof' meaning 'thief').

'arch-' entered English via Latin/Old French as a productive prefix meaning 'principal' or 'extreme', while 'thief' comes from Old English 'þēof' (from Proto-Germanic *þeubaz); the combination into the compound 'archthief' is a Modern English formation.

Initially simply denoting the 'principal' or 'chief' thief, it has been used figuratively to mean a particularly notorious or leading criminal; the core sense of 'chief' + 'thief' has remained stable.