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"to cheat, swindle," 1806, from dialectal duddle, diddle "to totter" (1630s). Meaning "waste time" is recorded from 1825. Meaning "to have sex with" is from 1879; that of "to masturbate" (especially of women) is from 1950s. More or less unrelated meanings that have gathered around a suggestive sound. Related: Diddled; diddling.
[cheating sense said to be fr Jeremy Diddler, a character in the 1803 novel Raising the Wind, by James Kenney]