'archmurderer' originates from English, specifically the combining prefix 'arch-' (from Greek 'arkhós') meaning 'chief' or 'principal' joined with 'murderer' (from Old English 'morthor' -> Middle English 'mordre'/'murder'), where 'arch-' meant 'chief' and 'murderer' meant 'one who kills unlawfully.'

'arch-' comes via Greek 'arkhós' into Late Latin/Old French combining forms and then into English as a productive prefix meaning 'chief' or 'principal'; 'murder' changed from Old English 'morthor' through Middle English 'mordre' to modern English 'murder', with the agentive suffix '-er' forming 'murderer'. The compound 'archmurderer' is therefore a modern English formation combining these elements.

Initially the components meant 'chief' and 'one who kills unlawfully'; combined in modern usage they denote an especially prominent or emphatic murderer (the sense is primarily emphatic/figurative rather than indicating a specific legal category).