From Longman Dictionary of Contemporary Englishgo mentalgo mentalBritish English spoken informal a) ANGRYto get very angry b) CRAZYto start behaving in an uncontrolled or excited way → mental
Examples from the Corpus
go mental• We're at the same position here as we were when we were selling out Harlow Square with the audience going mental.• Back home, the missus is going mental and your dinner's in the dustbin. 