Etymology
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clunk (v.)

1796, "to make the sound of a cork being pulled from a bottle;" imitative. This was the main sense through most of 19c. Meaning "to hit, strike" is attested from 1943 (perhaps a variant of clonk). Related: Clunked; clunking. As a noun, in reference to the cork-pulling sound, by 1823.

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clunker (n.)

"anything inferior," 1940s, agent noun from clunk (v.), probably in imitation of the sounds made by old machinery. Specific sense of "old car" was in use by 1936.

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clunky (adj.)

"blocky, ungraceful," by 1968 (when it was the name of a style of women's shoe), from clunk + -y (2). Related: Clunkily; clunkiness.

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