Yay
来自Big Physics
1960s: perhaps an alteration of yeah.
wiktionary
Alteration of yea(“yes; even, truly, verily”). More at yea.
From the sound it represents, by analogy with the other palatal letters chay and jay.
Clipping of yayo, from Spanish llello.
etymonline
yay
"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].
