Green’s Dictionary of Slang

up in (the) stirrups adj.

prospering, doing well.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 277: A man who is in swell street, that is, having plenty of money, is said to be up in the stirrups.
1821		‘Battle’ in Fancy I XVII 406: The Chatamites were all up in the stirrips at the success of their hero, and loudly offered to back him at 6 to 4.
1825		‘All England Are Slanging It’ Universal Songster I 40/1: The tyke-boys are all up in stirrups at the news.
1843		T. Haliburton Sam Slick in England II 131: Go and spend a week there; it will make you feel up in the stirrups.
1867		R. Whiteing Mr Sprouts, His Opinions 5: I says, stately like, to show I could be up in the sterrups too, ‘Look in the right-hand pocket of the coat, young man.’.