Though I'm not Korean, since I like this song and heard this word MANY times~, 'chu' is like a kissing sound. Does that make sense? Most of the time, when someone goes 'chuuuu~~', they have their lips pouted like they want to kiss someone. ^^ At least that's the only time I've ever seen 'chu' used. >w< I'm sure 'chu' has many different meanings because words in Korean may have more than one meaning depending on context, but I think this is the one you're talking about, especially in terms of this song~ Though, I think a native Korean may answer you better. ^^ This is just my two cents. :D *do it do it chuuu~*

I think Melody is right. I've never heard the song, and I'm not sure what chu is in Korean, but it sounds like they're using the Japanese onomatopoeia for a kissing sound, ちゅっ(Chu). The Korean onomatopoeia for kissing is actually 쪽 (Jjok). I think chu is cuter.