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