A type of old-fashioned or unreliable clock; possibly a clock with a loud or unpleasant striking sound.
From 'bum' (worthless, bad) plus 'clock.' The 'bum' prefix was used historically to indicate something inferior or defective, similar to 'bum steer' (false information).
The 'bum' prefix for 'worthless' is actually quite old—it appears in 19th-century slang for everything from 'bum fodder' (toilet paper) to 'bum freezer' (a short jacket), making 'bumclock' a natural construction for any unreliable timepiece.
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