'anti-climactic' comes from the adjective 'climactic' with the prefix 'anti-' (meaning 'against' or 'opposite'), where 'climactic' relates to 'climax' (the culminating point).

'climax' derives from Greek 'klimax' (κλίμαξ) meaning 'ladder' or 'stair'; it passed into Latin and then into Middle French/English. 'anticlimax' (noun) was formed by adding 'anti-' to 'climax' in the 17th–18th centuries; 'anticlimactic' as an adjective followed from that noun form.

Originally formed to indicate something 'opposed to' or 'contrary to' a climax (i.e., a descent from the expected peak); over time it has come to be used chiefly to describe an outcome that is disappointing or less exciting than expected.