"muscular low-I.Q. male," c.1910, perhaps from Spanish bozal, used in the slave trade and also to mean "one who speaks Spanish poorly." Bozo the clown was created 1940 at Capitol Records as the voice in a series of story-telling records for children ["Wall Street Journal," Oct. 31, 1983].
A fellow; a man, esp a muscular type with a meager brain: This bozo right here next to me
[Rep. Thomas P. O'Neill incognito] could probably be a better Congressman than those guys in Congress [1910+; origin unknown; perhaps fr Spanish bozal, used in the slave trade and after to designate someone who speaks Spanish badly, hence a stupid person]