"to cheat, swindle," 1806, from dial. duddle, diddle "to totter" (1632). 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.
tv. to feel someone sexually. (See also feel (so) up. Usually objectionable.) : She moved her hand over, like she was going to diddle him, then she jabbed him in the crystals.
in. to masturbate [oneself]. (Usually objectionable.) : Have you been diddling again?
tv. to masturbate someone else. (Akin to sense 1. Usually objectionable.) : She diddled him since it was his birthday.
tv. to cheat someone. : The shop owner diddled me out of ten bucks.
tv. & in. to copulate [with] someone. (Usually objectionable.) : I'm tired of hearing who has diddled whom in Hollywood.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw-Hill Education.
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1. To work with or modify in a not particularly serious manner. "I diddled a copy of ADVENT so it didn't double-space all the time." "Let's diddle this piece of code and see if the problem goes away." See tweak and twiddle. 2. The action or result of diddling. See also tweak, twiddle, frob. [Jargon File] (1995-01-31)