E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A butterfly man, a fop; so called from the popinjay or figure of a bird shot at for practice. The jay was decked with parti-coloured feathers so as to resemble a parrot, and, being suspended on a pole, served as a target. He whose ball or arrow brought down the bird by cutting the string by which it was hung, received the proud title of Captain Popinjay, or Captain of the Popinjay, for the rest of the day, and was escorted home in triumph. (See Old Mortality, ch. ii.)
I then, all smarting with my wounds being cold,
To be so pestered with a popinjay,
Answered neglectingly I know not what,
He should or he should not.
Shakespeare: 1 Henry IV., i. 3.
The Festival of the Popinjay. The first Sunday in May. (See above.)