From Longman Dictionary of Contemporary Englishbe mad about/for/on somebody/somethingbe mad about/for/on somebody/somethinginformal especially British English to like someone or something very much SYN crazy My nine-year-old is mad about Robbie Williams. He’s mad about computer games. All the girls at school are mad for him.be mad keen (on something) ‘Did you enjoy the film?’ ‘I wasn’t mad keen.’be mad for it (=want to do something very much) → mad
Examples from the Corpus
be mad keen (on something)• Some men are mad keen on football.• Giles was mad keen on planes from an early age. 