Tsun (ツン)= standoffish, mean, rude
Dere(デレ): fawning, lovestruck, caring
And so, if we put them backwards, we get a person who is loving and flirty at first but then becomes mean, rude and cold later.
Person one= Did you here? That girl is a deretsun
Persontwo= really? Who would've known. I just thought she was a deredere
A process of abnormally rapid physical, mental and emotional decline associated with severe, unusually unstable environmental conditions.
Whazza matter with that dude? Isn't he a scientist? He looks like he's lost the will to live...
Oh, he's got Accelerated Decrepitude just like almost every other sane intelligent person around here; hopefully he'll recover after the Mueller Report is issued...