to cheat or swindle someone out of money or valuables, or to waste time on pointless activities.
Origin uncertain, possibly from the name 'Dudley' or imitative of fiddle music. First recorded in early 1800s American English. The meaning shifted from playing an instrument to cheating, then to wasting time.
The word 'diddle' became slang so popular that it spawned 'diddly-squat' (meaning absolutely nothing), which is why you might hear someone say they got 'diddly-squat' in a bad deal—the word itself became shorthand for worthlessness!
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