IPA: /ˈhɒstlə/
KK: /ˈhɑːstlər/
A person who takes care of horses, especially at a place like an inn, or someone who services large vehicles like locomotives.
The hostler prepared the horses for the guests at the inn.
Hostler comes from the Old French word *ostelier*, meaning innkeeper or host, which itself derives from the Latin *hospitālis* (meaning of a guest or hospitality). The word refers to a person who provides lodging or care for guests, particularly in an inn or stable.
Think of someone who takes care of guests ('host') — that's why a hostler is someone who looks after those staying at an inn.