Also, like mad;like nobody's business. With exceeding enthusiasm or speed, without restraint. For example, We shopped like crazy and bought all our furniture in one day, or Once he's out of the town limits he drives like mad, or The choir sang the Hallelujah Chorus like nobody's business. The first terms employ crazy and mad in the sense of "lunatic" as a hyperbole for lack of restraint; the third implies that no business could be conducted in such an extraordinary fashion. The first and third date from the 1920s, the second from the mid-1600s.
American Heritage Idioms
Phrase(s): like crazy [and] like mad Fig. furiously; very much, fast, many, or actively. • People are coming in here like crazy. There isn’t enough room for them all. • We sold ice cream like crazy. It was a very hot day.
McGrawhill's American Idioms And Phrasal Verbs