'approx' originates as an abbreviation of the English word 'approximately', which in turn comes from 'approximate'.

'approximate' comes from Late Latin 'approximatus', the past participle of Latin 'approximare' (to come near), formed from 'ad-' (to, toward) + 'proximus' (nearest). The English 'approximate' developed through Old French/Anglo-Norman influences into Middle English, and 'approx' later arose as a clipped written form of 'approximately'.

Initially based on Latin meaning 'to come near', it developed into the English sense 'near, not exact' and the clipped written form 'approx' retains that sense of 'about/roughly'.