horseshoe up one's ass en · NOUN
Etymology
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.
Meanings
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(idiomatic, vulgar) Exceptionally favorable personal luck.
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.
2010 July 8, "Dave Mustaine's Autobiography Tells 'A Story That Will Inspire, Stun And Terrify', blabbermouth.net (retrieved 1 Feb 2019)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.
2014, Penelope Ward, Stepbrother Dearest, →ISBN:But Mary Ziganetti shook her head. "Teresa never had the luck. Some people, they got a horseshoe up their ass, but not Teresa."
2001, Louisa Ermelino, The Black Madonna: Una Storia di Famiglia, →ISBN, page 36: