'unflatteringly' originates from English formation: the prefix 'un-' (from Old English 'un-') meaning 'not' combined with 'flattering' (adjective form of the verb 'flatter'). 'Flatter' comes from Middle English 'flater(en)', from Old French 'flater' meaning 'to praise or cajole'.

'unflatteringly' developed by adding the negative prefix 'un-' to the adjective 'flattering' (from 'flatter' + '-ing'), then adding the adverbial suffix '-ly'. The chain is roughly: Old French 'flater' → Middle English 'flater(en)'/'flatter' → English 'flatter' → 'flattering' → 'unflattering' → 'unflatteringly'.

Originally 'flatter' meant 'to praise (often insincerely) or to cajole'; over time 'unflatteringly' evolved to mean 'in a way that does not flatter' — i.e., presenting someone or something unfavorably or critically.