A tsundere is a male or female that is mean or cold to their love interest (it could also be a friend) because they don’t know how to express their feelings. You could say that they “Hate” that they’re in love with you. A good example of this could be kageyama from haikyuu. When Hinata does something good kageyama finds it hard to compliment him and usually just looks away from hinata. Another example is osana najime from the game yandere simulator. Tsunderes tend to use the words baka, dummy, idiot, or stupid A LOT and I’m not over exaggerating when I say a lot.

Tsun tsun means to turn away and put you nose up in embarrassment or digust

A cute ass boy or girl who doesn’t know how to express their feelings to you so they are mean instead