A loud, resonant sound, especially a metallic one; also informally means a notable mistake or blunder.
From the verb 'clang' plus the agent suffix -er, originally describing something that makes a clanging sound; the 'mistake' meaning developed in 19th-century slang.
The shift from 'loud sound' to 'terrible mistake' might have come from sports—when an athlete made a big error, commentators said they 'dropped a clanger' like a dropped metal object.
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