One of the meanings of diddle is to fool around in an unproductive manner. This is not the same as brainstorming.
"I promised myself I was going to clean the apartment, but I'm afraid I just ended up diddling around."
Diddle can also mean to move back and forth, either literally or metaphorically.
"This key is really badly cut. Every time we use it, we have to diddle the tumblers forever."
"That's not a recipe, I just had some chicken and lentils and onions, and diddled with the spices until it tasted right. But I'll write the idea down for you, if you like."
This is a very straightforward meaning; your usage of it may be accompanied by profanity and followed by legal action.
"The man diddled me out of my entire retirement fund -- $500,000."
Many of us can recite from memory the nursery rhyme that begins "Hey diddle, diddle, the cat and the fiddle...." These words may also fill our heads with enchanting imagery. The military saying, "Hey diddle, diddle, straight up the middle!" is the infantryman's way of describing a straightforward, violent assault into a prepared enemy position. No tactical finesse is involved, high casualties are expected and irony is a coping mechanism.