Originating on the Jersey Coast. Means 'very much so', or 'a whole lot'. Used after a description or declarative part of a sentence to add feeling.
Man, that band rocks like mad.
Derek must have worked like mad to get that report done on time.
Man, that run was hard. I'm sweating like mad.
That guy at the model train show was a dork like mad.
A term used as sarcasm and to give someone an attitude when someone is wrong.
Guy: Wanna go out tonight?
Girl: LIKE MAD! *slaps guy in the face*