From 绷不住了, bēng bù zhùle, literally "unable to stretch anymore". Means to be pent up and unable to hold back (such as in sexual urges, laughter, or anger). Basically means, "I can't help it".
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 barlast night and some asshole was undenging me.
4: Man, you are so undeng.