'unphonetic' originates from English, specifically the prefix 'un-' and the word 'phonetic', where 'un-' meant 'not' and 'phonetic' ultimately comes from Greek 'phonētikos' meaning 'relating to sound'.

'phonetic' changed from Greek 'phonētikos' to Latin 'phoneticus' and into Middle English as 'phonetic'; the negative prefix 'un-' (from Old English 'un-') was attached in Modern English to form 'unphonetic'.

Initially it meant 'not phonetic' and over time it has retained that basic meaning of 'not reflecting pronunciation'.