To be 'undeng' is to be of an exemplar brand of self-defecating fool. First syllable being a cut-short "under" and the second syllable being a play on the word "dung". Underdung; undeng; deep shit. It is also used to describe someone who is blissfully ignorant.
As such, it is assumed that whoever assumes these qualities is of a social demeanor that puts them in "deep shit", as the root of the phrase suggests.
Because of the phrase's malleability, it can be used both to describe a negative action done to another and it can be used to describe what a person is or how they are acting.
1: Don't undeng me, bro.
2: Why are you being so undeng?
3: Dude, I went to the bar last night and some asshole was undenging me.
4: Man, you are so undeng.