Unlikely as it may sound, I have recently been startled to find myself prone to exclaiming “YAY!”, for which I’m about forty years too old.
This sad habit isn’t formed so much by watching the brilliantly awful Jessica Hynes on the BBC TV series W1A (see this post on voiceovers), as by finding the ironic ambivalence of its usage a useful shorthand. Once it might have been “YES!”, and still earlier “Jolly good show!”, “Hoorah!”, or “Spiffing!”, but “YAY!” has more layers of meaning—the older the speaker, the more pleasantly disquieting it sounds. So that’s my defence.
Chinese vocabularies for approbation change even faster. Victor Mair’s language log has some fine examples, like this. Following bucuo and taibangle, I still like the north Shanxi kebulai—but I’m not yet aware of any terms with the ironic sense of “YAY!”.