YAY

Definition

"Used as an exclamation of pleasure, approval, elation, or victory.
Dictionary.com

Etymology

Yes
O.E. gise, gese "so be it!," probably from gea, ge "so" (see yea) + si "be it!," third pers. imperative of beon "to be" (see be). Originally stronger than simple yea. Used in Shakespeare mainly as an answer to negative questions. Yes-man is first recorded 1912, Amer.Eng. - www.etymonline.com
Yea
O.E. gea (W.Saxon), ge (Anglian) "so, yes," an affirmative word in Gmc., cognate with Ger., Dan., Norw., Sw. ja. Spelling yeah first recorded 1905. - www.etymonline.com


Yay is an alternate spelling of the word Yea (orginally Gea). Gaea (Gaia) is the Greek goddess personifying the Earth. In Sumerian, she is called Ki. A Geas in Celtic mythology is a vow or obligation put upon a great hero. Typically, the geas would offer great benefit if obeyed, and great tragedy if broken.

Yes -> Yea (Yay) -> Gea -> Geas <- Gaea (Gaia | Ki)