'clunker' originates from English, specifically the word 'clunk' (an imitative/onomatopoeic term), with the agentive suffix '-er' meaning 'one that makes a clunk or is associated with clunking'.

'clunk' is imitative of a heavy dull sound (recorded in earlier English), and 'clunker' developed by adding '-er' to form 'one that clunks' or 'a thing that makes a clunk'; by the early 20th century it was used informally for old or malfunctioning machinery and then for old cars.

Initially it referred to something that made a 'clunk' (a heavy dull sound) or a heavy object; over time it specialized to mean 'an old or unreliable car' and figuratively 'a failure/dud'.