Green’s Dictionary of Slang

horseshoe n.

[the trad. association of horseshoes and luck]
(US)

1. as horsehoes, a nickname for a very lucky person.

[US]R. Lardner ‘Horseshoes’ in Coll. Short Stories (1941) 256: You got a swell girl, Dick! [...] You’re mighty lucky to cop her out — you old Horseshoes, you!

2. a propensity for good luck.

[US](con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 204: You got too many horseshoes.
[US]E.F. Frazier Negro Youth xvii: ‘As soon as we can get you on the [tennis] courts we want you. We haven’t forgotten the ‘horse shoes’ you had last year. Too bad you won’t try to take it out on me in Ping-pong. On a table I’m a champion and I know it.’.

3. (Aus.) a bar counter.

[Aus]D. Whish-Wilson Line of Sight [ebook] There were a few dockworkers leaning on the horseshoe.

In phrases

have a horseshoe up one’s ass (v.)

to be notably lucky.

[US]E. Torres Carlito’s Way 56: He was made in the thirties, always seemed to be on the winning side in all the wars, a survivor. [...] A crazy ginso with a horseshoe up his ass.