If we take {qIvon} to mean "knee" as the writers intended, for instance as a variant from Kor's native dialect, Dax's use of it would also emphasize that she's Curzon, since someone who didn't know Kor that well might just say {qIv}. If we also take {qIvon} as a standard dialect word for a possibly-vaguely-naughty body part, as suggested by the way Maltz talks about it, it would also emphasize the friendship between Curzon and Kor, because making innuendos and double-entendres seems like the sort of thing Kor and his warrior buddies would get up to.

It's also possible that {qIvon} is sometimes used as a sort of bawdy slang replacement for {qIv}, because of the phonetic similarity.