'antiflattering' is formed by the prefix 'anti-' (from Greek 'antí' meaning 'against') combined with 'flattering' (from the verb 'flatter', from Middle English 'flattern', from Old French 'flater').

'flatter' changed from Old French 'flater' into Middle English 'flattern' and eventually modern English 'flatter'; the prefix 'anti-' was borrowed from Greek via Latin and has been productively attached to English adjectives to form compounds like 'antiflattering'.

The compound originally carries the literal sense 'against flattering'; in modern usage it simply describes something that is 'not flattering' or presents an unfavorable impression.