yayyoudaoicibaDictYouDict[yay 词源字典]
"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].[yay etymology, yay origin, 英语词源]