The phrase comes from British slang where wet is used to mock someone as soft, lame, or acting pathetic, and u man r wet is written in texting shorthand for you lot are wet. Short shouted versions of the line spread through soundboards and meme pages as a reusable insult clip rather than from a single film or song. Its popularity is down to the exaggerated delivery, which makes it instantly recognisable even in text form and easy to drop into gaming, football banter, and reaction edits without any setup.