This deeply insulting word for a vain or conceited person, one given to pretentious displays, is now rather dated or literary. A good example can be found in Joseph Conrad’s short story The End of the Tether of 1902: “When he looked around in the club he saw only a lot of conceited popinjays too selfish to think of making a good woman happy”.
Dictionaries say a popinjay was also at one time the usual name for a parrot, and in that lies the origin of the derogatory term. What could be more gaudily and squawkingly in your face than a parrot? What more perfect term for an empty chatterer, fop or coxcomb? Who’s a pretty boy, then?
It’s an ancient imprecation, already of some age when Shakespeare used it in Henry IV, but the literal parrot sense goes back even further, to the latter part of the fourteenth century. It was also used for a device on a post to shoot at, the archers’ equivalent of the quintain, usually it seems because the mark was a figure of a parrot. That explains references such as this one, in Old Mortality, by Sir Walter Scott: “When the musters had been made, and duly reported, the young men, as was usual, were to mix in various sports, of which the chief was to shoot at the popinjay, an ancient game formerly practised with archery, but at this period with fire-arms”.
The word travelled with the bird from Africa and can be traced back to the Arabic babbaga, through Spanish papagayo and Old French papeiaye. One of the earlier English versions (it had lots of forms before it settled to the spelling we know now) was papengay but it seems the ending was changed because people thought the name referred to a sort of jay.