Definition & Meaning of "of course my horse"

What is the origin of the idiom "of course my horse" and when to use it?

The idiom "of course my horse" is believed to have originated in early 20th century American English horse racing culture. When bettors would enthusiastically agree with each other about a particular horse winning a race, they would say "of course my horse". It is used to imply full confidence in and support for their chosen contender.