A thunderously loud (hence the name) stream of piss that is unleashed in either of the following two situations:

1) When you first wake up in the morning and have to piss so badly that you feel like your bladder is about to explode;

2) After you have consumed an abundance of alcohol in a relatively short period of time.
1) John woke up this morning, immediately staggered into the bathroom and proceeded to cut loose a devastating stream of thunderpiss.

2) After Frank polished off the coffee pot full of whiskey and coke in less than an hour, he sprayed the sidewalk with thunderpiss.
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Steve: "Mary Ann, I'm going to take a thunderpiss on our electric fence."

Mary Ann: "Have fun, swee -- wait, what?"

Steve: *background* "OH MY GOD!!!!!"
by Exoras July 7, 2006
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