1603, in the meaning defined above
borrowed from French & Latin; French fabulosité, going back to Middle French, borrowed from Latin fābulōsitāt-, fābulōsitās, from fābulōsus "celebrated in legend, resembling an invented story, mythical" — more at fabulous
Note: Late 20th- and 21st-century use may represent a direct derivation from fabulous, after curious : curiosity, generous : generosity, and like pairs.