I’ve made an astonishing etymological deduction….studying the rather more seriously good pictures that line the walls of the Garrick Club, that luvvie-dovey haven of literary London, the other night, I came across this gem.

The shear outrageousness of his greenery-yallery costume surely implies a certain……well, gaiety…… and when one reads his name on the frame, let alone the part he was playing.

It seems to me it’s pretty obvious that this is the origin of that much-discussed word.