"Jack's Mum" is one of the more common variations of the classic Your Mum Joke.
Originating in the Grimsby area of England, the phrase has quickly circulated, becoming common among teenagers.
Although its implementation and purpose reflect that of the Your Mum Joke, it can be used to make yourself look cool and witty, while not directly insulting anyone. Commonly referred to as "The Gentleman's Your Mum Joke".
Person 1: Hey, if you eat too much you'll get sick.
Person 2: That's what I told Jacks Mum last night.
Person 1: Egg Fried Rice.
Person 2: Thats what Jack's Mum said.