On a fiddle diddling is playing fast. If you're diddling on a wind instrument then you're probably using the front and back parts of your tongue alternately to separate one note from the next (particularly useful when tootling would create too much air pressure for the low notes to sound cleanly.)
Diddling is akin to fiddling in another sense, too: if you're doing something fiddly, like trying to get a too-big nutmeg out of a jar, then you might in your frustration give the thing a quick diddle - that is, shake it violently up and down - in the hope that that'll make the thing come out.
It won't, though it might relieve the feelings. The only thing to do with the nutmeg is either to smash the jar, or drill bits off the nutmeg. That nutmeg has been forced in, and nothing will persuade it to come out again.
(I speak from bitter experience.)
If your problem is not caused by a nutmeg in a jar but a pig in a poke then I'm afraid you've been diddled: that is, cheated by being given a promise that hasn't been fulfilled. Which, come to think about it, is also called being fiddled, isn't it.
Lastly we have the originally North American term diddly-squat. According to my Collins dictionary, in America diddly-squat means anything, as in that don't mean diddly-squat; but in Britain I've only ever heard it used to mean nothing, as in the agents get a £50,000 fee and the writers get diddly-squat.
Hm. I'm beginning to wonder if my dictionary is right. Perhaps someone from North America would let me know.
Ah well, at least everyone understands each other.
That doesn't always happen, you know.
Thing Probably Not To Do Today: diddle. This word is probably a variant of doderen, to totter or tremble. The cheating sort of diddle is named after Jeremy Diddler:
a scrounger in J Kennedy's 1803 farce Raising the Wind.