gee¹ / jē/ (also gee-whiz / ˈjē ˈ(h)wiz/ ) • interj. inf. a mild expression, typically of surprise, enthusiasm, or sympathy: Gee, Linda looks great at fifty!gee² • interj. (gee up) a command to a horse to go faster.• v. (gees, geed, gee·ing) [tr.] command (a horse) to go faster. ∎  encourage (someone) to work more quickly: I was running around geeing people up.gee³ • n. inf. a thousand dollars: we paid five gees.

gee int. word of command to a horse. XVII.
Hence as sb. esp. redupl. gee-gee (XIX) child's name for a horse.