Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
show off (v.), showoff (n.)
The verb has two Standard senses, a transitive, to display for others admiration, as in He showed off his new pony, and an intransitive, to make a vulgar display of self, as in She showed off so shamelessly that we all hated her. A showoff (stress on show-) is just such an exhibitionist.