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An event when an individual tries to urinate and comparatively insignificant amount of urine is expelled out of the body (less than 50 ml usually).
Example 1:

Man to a Friend: I woke up last night really needing to take a piss. After forcing my tired ass to the loo only a few drops of piss came out.

Friend. It was only a pee-diddy. Don't worry about it.

Example 2:

"I usually end up doing a pee-diddy just before I undertake a sporting activity. I just have to get all the liquid out of me to feel comfortable on the field."
by StNaz August 30, 2011