'letdown' originates from English, specifically the phrasal verb 'let down', where 'let' comes from Old English 'lǣtan' meaning 'to allow/leave' and 'down' comes from Old English 'dūn' meaning 'downwards'.

'let' changed from Old English 'lǣtan' and 'down' from Old English 'dūn'; the phrasal verb 'let down' developed into the compound noun 'let-down' in the 19th century and later the closed form 'letdown'.

Initially it meant 'to lower or let something down' (physical sense), but over time it evolved into the figurative sense 'to disappoint', and the noun 'letdown' came to mean 'a disappointment'.