E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Gee-up! and Gee-woo!
addressed to horses both mean Horse, get on. Gee = horse. In Notts and many other counties nurses say to young children, Come and see the gee-gees. There is not the least likelihood that Gee-woo is the Italian gio, because gio will not fit in with any of the other terms, and it is absurd to suppose our peasants would go to Italy for such a word. Woa! or Woo! (q.v.), meaning stop, or halt, is quite another word. We subjoin the following quotation, although we differ from it. (See COME ATHER, WOOSH.)
Et cum sic gloriartur, et cogitres cum quanta gloria duceretur ad illum virum super equum, dicendo Gio! Gio! cepit pede percutre terram quasi pungeret equum calcaribus.Dialogus Creaturarum (1480).