The word baka (ばか / 馬鹿) is a Japanese word that means idiot, fool or silly (depending on context). It is also used in the English language, especially among fans of anime (and subcultures in close contact) for the same meaning.
When written with Chinese characters, baka is spelt like "horse-deer". A legendary etymology for why these characters were chosen tells of a Chinese emperor who spotted a horse (馬). He called it a deer (鹿) and all but one of his advisors agreed. The dissenting advisor proved that it really was a horse and the emperor executed his other sycophant advisors and raised the correct one to chief advisor.
It should be noted that this is a Japanese story, not a Chinese one. It seems to be a misinterpretation of a well-known story behind a Chinese idiom, "Point at a deer and call it a horse," which means to confound right and wrong intentionally.
When used as an adjective in Japanese, baka (na) can mean "absurd" or "crazy", not necessarily "idiotic".
Baka is also a tribe of Pygmies.