horseshoe up one's ass

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 is a traditional good luck  in. 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.

Noun

 * 1)  Exceptionally favorable personal luck.
 * 2) * 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.
 * 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.

Usage notes

 * Often used in contexts which suggest resentment of the favorable luck so described. In this respect, similar to : it implicitly deprecates the person so described, connoting unearned assets, and the term would generally not be used as self-description.