To mug someone off is to disrespect them, stitch them up, or make them look like a proper mug, usually by chatting rubbish, flaking, cheating, or doing them dirty in public. It got a big boost from Love Island, but it was knocking about before that. Honestly, it’s the perfect phrase for petty betrayal.
"He said he was skint then posted a story buying shots for the whole pub. Nah, he’s mugged you off, mate. Block him and get a kebab, you deserve peace."