According to http://www.word-detective.com/0806A.html , 'many small children had noticed that a lot of grownups were shouting "gee" at horses and decided that "gee" was another name for "horse."' but according to http://en.wikipedia.org/wiki/Ottawa_Gee-Gees , 'The term "Gee-Gee" is taken from horse racing where a Gee-Gee is the first horse out of the starting gate.' Are these correct?
Learn English – Why is “gee-gee” slang for “horse”
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Best Answer
This Wikipedia article gives this explanation for the origin of the word gee-gee:
This article expands on that:
This definition of gee-gee says it is a UK child's word for horse. I don't believe that is used by children in the U.S.
I don't know if this is relevant, but gee or gee up is also a command to get a horse to move faster. (It is also used to have a draft animal turn right. As opposed to haw, which is a command to turn to the left.)
From "Horse racing jokes" on http://www.ukjockey.com/jokes.html