Why are clowns called clowns?

Well I think that being a clown means like being goofy, silly. So clowns are called clowns because they are silly.

Not really! Clowning is a very difficult art to perfect just being silly, anyone can do that.
'Clowning is the universally accepted name for it but to be really accurate the 'clown' is the white face character not used very much any more. He is the talented musician and feed. The 'clowns' are really the stooges and the 'Augustes' who are the fall-a-bout acrobats and the laughter makers. Modern clowns who dress too much and have too much (frightening?) make up are not the traditional fall-a-bouts such a Stan Laurel or Chaplin. Ragged costumes and nonsense props are all a part of good slapstick. They should not frighten children!