Going to the toliet to urinate, in reference to a horses trough so the term only applies to men.
"Yeah sorry lads, have to go see a man about a horse"
by Aaron Fielding January 30, 2008
A polite way of saying that you need to take a dump
"Be right back...I gotta go see a man about a horse"
by Cheese Steak Killer October 13, 2007