IPA: //ˈreɪnz.mən//
KK: /ˈreɪnzˌmæn/
A person, usually a man, who drives or rides horses, especially in races or competitions.
The reinsman skillfully guided the horses around the track during the race.
Reinsman → It is formed from "rein" (from Old French *reine*, meaning a strap or cord used to control a horse) and "-sman" (meaning a person who). A reinsman is a person who controls a horse using reins.
Think of someone who uses a 'strap' ('rein') to control a horse — that's what a reinsman does.