Baka

Baka is frequently used term in anime and manga and basically means "fool", "idiot", "moron", "stupid", or some other derogatory label for poor behavior or a person who behaves poorly. It can vary in level of insult depending on the context. It can even be used as an affectionate (if not super positive) nickname.

"Baka" is used mostly in the Tokyo region and surroundings, but other terms may be used in other regions of Japan (as opposed to aho in the Kansai region). In the Kansai region, "baka" is often seen as only negative connotation.