When something is really cool, fun, or awesome.
In French "il pleut" means "it's raining" so if something is dripping in fun or coolness or is so awesome it's raining awesomeness, then "il pleut"
Tyler: dude how about this party
Avery: oh yeah il pleut
Kai: il pleut af
Lucille: did you hear the new Nicki Minaj song?
John: oh yeah! Il pleut!
a joke phrase said, meaning "when it rains, it rains" in French. another way to poke fun at people with a sad bart profile picture.
Joe: "my mom just died."
Dave: "well, you know what they say."
Both: "quand il pleut, il pleut."