'downturn' is a compound formed from the adverb 'down' and the noun/verb 'turn'. 'down' originates from Old English 'dūn' meaning 'downward, away', and 'turn' comes via Old French 'tourner' from Latin roots meaning 'to revolve or change direction.'

'downturn' developed as a compound in modern English (recorded from the late 19th to early 20th century) to describe a 'turning downward' in condition or fortunes; the sense was extended metaphorically to economic and business contexts during the 20th century.

Originally a literal sense of 'a turn downward' (physical or directional), it evolved into the figurative sense of 'a decline or worsening of conditions,' especially in economic usage.