Twankle
To twang your fingers on a musical instrument or absent-mindedly strum or play an instrument without thought.
Other concatenations include: Twiddling, twandling, tootling, plunking, noodling, thrummling or tudeling.
Tudeling (origin) German dudeln – to perform badly.
The crappiest song that almost everyone can play on the piano Chopsticks was invented in 1877 by composer Euphemia Allen.
Read more: The Cabinet of Linguistic Curiosities by Paul Anthony Jones

Thought this was going to a word for ‘adults’…but instead it’s something I’ve been known to do!
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Hehehe now that you say that, twankle from the sound of it could be an ‘adult’ word.
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