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When someone takes your stuff and puts it on Facebook like it's their own, and you're so mad you type 'I showed you that' in their post just to annoy them. It's usually for stuff everyone already knows, so you're basically saying 'You didn't even give me credit, you lazy piece of garbage.'
'I showed you that', said by a guy after his friend posted a meme he made 2 years ago.
A girl comments 'I showed you that' on a post about the same cat meme she posted 3 days ago.
A guy DMs his best friend 'I showed you that' after he posted a link the guy gave him 4 hours ago.