What does this idiom mean......ive got to talk a man about horse?

The idiom you're referring to is "I've got to talk to a man about a horse." It's a euphemism for needing to use the restroom, specifically for men.

Here's why it's a euphemism:

* "Horse" is a slang term for the toilet or urinal.

* "Talk to a man about" is a vague and indirect way of saying "use."

So, essentially, the idiom is a polite way of saying "I need to go to the bathroom."