To have something up one's ass (= stowed there) is (in cynical jocular formulation) to have it on hand and available despite the unlikelihood of being so equipped. (You're going to patch that tire right now, out here? What, do you have a patch kit up your ass?) A horseshoe is a traditional good luck charm in folklore. Thus, to have a horseshoe up one's ass is to have an unlikely amount of luck and to carry it around continuously in a way that seems unexpected.

horseshoe up one's ass

1. (vulgar, idiomatic) Exceptionally favorable personal luck.
   • 2001, Louisa Ermelino, The Black Madonna: Una Storia di Famiglia, →ISBN, page 36:

     But Mary Ziganetti shook her head. "Teresa never had the luck. Some people, they got a horseshoe up their ass, but not Teresa."

   • 2010 July 8, "Dave Mustaine's Autobiography Tells 'A Story That Will Inspire, Stun And Terrify', blabbermouth.net (retrieved 1 Feb 2019):

     METALLICA frontman James Hetfield once observed that Mustaine must have been born with a horseshoe up his ass. That's how lucky he has been.

   • 2014, Penelope Ward, Stepbrother Dearest‎^[1], →ISBN:

     The next thing I knew, she'd run off to the roulette table, slapped some of her money down on the number 22, and the rest was history. This girl had a major horseshoe up her ass. Nineteen-thousand dollars.

• Often used in contexts which suggest resentment of the favorable luck so described. In this respect, similar to born with a silver spoon in one's mouth: it implicitly deprecates the person so described, connoting unearned assets, and the term would generally not be used as self-description.

• pull out of one's ass