'arch-liar' is a Modern English compound formed from the prefix 'arch-' and the noun 'liar'. 'arch-' ultimately comes from Greek 'arkhós' (via Latin/Old French), where 'arkhós' meant 'chief' or 'principal'; 'liar' comes from Old English 'lēogere' (from the verb 'lēogan') meaning 'one who lies'.

'liar' changed from Old English 'lēogere' (from the verb 'lēogan') to Middle English forms like 'lier' and eventually to the modern English 'liar'. The prefix 'arch-' entered English via Old French/Latin from Greek 'arkhós'. The compound 'arch-liar' is a later Modern English formation combining these elements to mean a principal or extreme liar.

Initially the prefix 'arch-' meant 'chief' or 'principal', and used with 'liar' would denote the 'chief liar'; over time the compound has been used emphatically to mean an 'extremely dishonest person' or a 'notorious liar'.