Balls!
Or rather: their absence then.
In Germany, a
Stier is fertile male cattle, while neutered males are
Ochsen. We also have the word
Bulle for fertile male older cattle,
Stier is generally associated with a more youngish animal, but the differentiation isn't always that clear. All bulls 'fighting' Spanish matadors (i.e. being abused by them) are
Stiere to us, bullfighting is
Stierkampf.
In our law firm, the young associate who mistakenly drafted a horse sales contract obligating the seller to warrant fertility of the
Wallach (= gelding) to the buyer is the stuff of mythological legend. Though Georg, once he was made aware that he was asking for the impossible to be warranted, gave as good as he got: "
See, that is typically wasteful Western capitalism, I'm from East Germany, we had nothing, but lots of it, and in communism you learned to never throw anything away!"
Georg's equestrian knowledge might not have been all that sure
footedhoofed, but he became a very successful attorney/partner and eventually the managing partner of the Frankfurt office of a US competitor. He still tells the story of the "
zeugungsfähiger Wallach" (fertile gelding) to his new clients to break the ice.