yay

interjection Informal.

1. (an exclamation used to express joy, excitement, etc.)

adverb Informal.

1. to the extent, amount, etc., indicated: The doll is about yay high.

interjection

1. informal an exclamation indicating approval, congratulation, or triumph

“this,” as in yay big “this big,” 1950s, perhaps from yea “yes” in its sense of “even, truly, verily.” “a sort of demonstrative adverb used with adjectives of size, height, extent, etc., and often accompanied by a hand gesture indicating size” [DAS].