Definition 1: A reply to be used when someone thinks they have the answer to a question but another individual participating in the argument proves them wrong with a detailed and knowledgeable answer with no sympathy for the other original participant's dignity or reputation.
Definition 2: A reply to be used when someone replies to an insult or argument with a flawlessly epic one liner or put down.
Think of an absolutely awesome party abruptly and absolutely ruined by the arrival of your grandfather along with his comments about the insolent and hopeless youth. Well that's a royally grandpa-pee'd moment.
Well a grandpa-pee'd moment is pretty much obvious, you cannot miss it when it strikes down on you.
Confusing expression of love/admiration for a woman. Similar to boys pulling a girl's hair on the playground. He really cares for her, but is uncomfortable/awkward about expressing the emotion. This euphemism that sounds vaguely degrading is known to mean that he's actually really in to her.
Can also be used in the accusatory "He'd pee on her porch" - used to call out a "bro" for his love without outright embarassing him and having uncomfortable conversations about real feelings, which should, obviously, be avoided at all times.
"That woman has duel masters degrees, volunteers at the soup kitchen, and has the body of a goddess. I'd pee on her porch"
"Look at Gifford talking to that straight up supervixen, he'd pee on her porch"