The word Baka (馬鹿) is the Japanese word that means idiot, fool or silly (depending on context). It is also used in the English language among fans of Anime (and subcultures in close contact) for the same meaning.
Baka essentially translates to "horse-deer". The story behind baka is of a Chinese emperor who spots a horse (馬). He calls it a deer (鹿) and all but one of his advisors agree. The dissenting advisor proves that it really is a horse and the emperor executes his other sycophant advisors and raises the correct one to chief advisor.
It should be noted that it 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.
Baka is also a tribe of Pygmies.