'dorso-posteriorly' is a modern English compound formed from the combining form 'dorso-' (from Latin 'dorsum' meaning 'back') + 'posterior' (from Latin 'posterior', comparative of 'posterus' meaning 'coming after, behind') with the adverbial suffix '-ly' (from Old English/West Germanic '-lic' evolving to '-ly').

The elements come from Latin: Latin 'dorsum' gave the combining form 'dorso-' used in anatomical compounds; Latin 'posterior' passed into English via Medieval Latin/Old French as 'posterior'. The adjective 'dorso-posterior' is a 19th–20th century medical/ anatomical compound, and the adverb 'dorso-posteriorly' was formed by adding the English adverbial suffix '-ly'.

Originally the roots referred separately to 'back' and 'behind'; over time they were combined in technical anatomy to specify a single compound direction ('toward the back and rear'), and this specialized directional meaning has been preserved in modern usage.