too big for one's britches (breeches)

Conceited, self-important. This expression, alluding to becoming so swelled with self-importance as to burst out of one’s clothes, sounds ancient but dates only from about 1900, as does the closely related too big for one’s boots. The latter appeared in Sir Henry Maxwell’s Life of W. H. Smith (1894): “Sometimes a young man, ‘too big for his boots,’ would sniff at being put in charge of a railway bookstall.” And H. G. Wells (Kipp, The Story of a Simple Soul, 1905) wrote, “He’s getting too big for ’is britches.”