Mad  adj. (madder, maddest) 1 insane; frenzied. 2 wildly foolish. 3 (often foll. By about, on) colloq. Wildly excited or infatuated. 4 colloq. Angry. 5 (of an animal) rabid. 6 wildly light-hearted.  like mad colloq. With great energy or enthusiasm.  madness n.  [old english]