meaning – What exactly does it mean to “mug somebody off” in British …

It may not have any well-defined meaning as a phrase (I’m British, and I’ve never heard it). “Mug” has a variety of uses, so a sentence like that is probably not an idiom at all. You should bear in mind that a film may go to some lengths to accentuate (or even make up) “cockney-isms” — don’t mistake movie Cockney for British English, they are …

https://english.stackexchange.com/questions/20477/what-exactly-does-it-mean-to-mug-somebody-off-in-british-english