"do kup ngaa"?

[Diane]

This has something to do with teeth... any ideas/translations? Differences w/ "sau ngaa"? Thanks!
-Di

[Terence]

dou3 kap6 ngaa4 ~ a person with his lower jaw protruding more than the upper. i.e. lower row of teeth covers the upper when shutting up mouth. thus he always has a protruding chin.