1 [with object] cheat or swindle (someone) so as to deprive them of something:he thought he’d been diddled out of his change
deliberately falsify:he diddled his income tax returns
2 [no object] chiefly North American pass time aimlessly or unproductively:I felt sorry for her, diddling around in her room while her friends were having a good time
3 [with object] vulgar slang, chiefly North American (of a man) have sexual intercourse with.
[originally in Scots dialect use in the sense 'jerk from side to side', apparently corresponding to dialect didder 'tremble']