Stop paying attention, dissociate oneself from a situation. Also, engage in a mindless activity. For example, When Felicia starts talking about her ailments and her friends' ailments, I totally zone out. This idiom also occurs in the passive, be zoned out. It originally alluded to narcotic intoxication and then was broadened to other kinds of dissociation. For a near synonym, see tune out, def. 2. [Slang; second half of 1900s]