A dere-tsun is the opposite of a tsundere
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
Person
two= really? Who would've known. I
just thought she was a deredere