Too big for your breeches
Conceited; having a too high opinion of oneself.
'Too big for your breeches', or 'too big for your britches', sounds like an American phrase, and it is. It is first found in print in An Account of Col. Crockett's Tour to the North and Down East, 1835, written by Davy Crockett.
I myself was one of the first to fire a gun under Andrew Jackson. I helped to give him all his glory. But I liked him well once: but when a man gets too big for his breeches, I say Good bye.
Those of a certain age will probably best recall Davy Crockett as 'the king of the wild frontier' from the 1950s television show and put him in the same category as fictional folk heroes like the Cisco Kid and the Lone Ranger. Crockett was, however, a real life US politician who represented Tennessee in the U.S. House of Representatives.
A later alternative version of the phrase - 'too big for his boots' is found in both the USA and the UK from the 1860s onwards and may have originated in either place. The first example that I have found comes from the pen of the Scots writer Laurence Lockhart in the novel Doubles and Quits, which was serialised in Blackwood's Edinburgh Magazine in 1868:
I could scarcely repress an exclamation of wrath and disgust when I saw him lolling familiarly in my arm-chair. He was getting too big for his boots; and then his abominable tobacco and whisky - faugh! it was insufferable.
One spin-off phrase that is undoubtedly British and is unlikely to be claimed by anywhere else is 'I'm too sexy for my shirt', the title of the 1991 song by the UK trio Right Said Fred (rated number 49 in Blender Magazine's 50 Worst Songs Ever Recorded poll).
In a piece that mentions Davy Crockett, it would be remiss not to include the old joke: 'Davy Crockett had three ears; a left ear, a right ear and a wild front ear'. (Number 49 in the Phrasefinder 50 Corniest Jokes Ever Inflicted poll).