A nickname of Prince George of Denmark, given him by James II. The story goes that James, speaking of those who had deserted his standard, concluded the catalogue with these words, “And who do you think besides? Why, little Est-il-possible, my worthy son-in-law.” James applied this cognomen to the prince because, when George was told of his father-in-law's abdication, all he did was to exclaim,
“Est-il-possible?” and when told, further, of the several noblemen who had fallen away from him,
“Est-il-possible?” exhausted his indignation.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894