GOOFBALL
clown, buffoon, goof, goofball, merry andrew
(noun) a person who amuses others by ridiculous behavior
fathead, goof, goofball, bozo, jackass, goose, cuckoo, twat, zany
(noun) a man who is a stupid incompetent fool
Source: WordNet® 3.1
Etymology
Noun
goofball (plural goofballs)
(informal, often derogatory, sometimes affectionate) A foolish or silly person or animal.
(informal, pharmaceutical drug) A pill or tablet containing a pharmaceutical which has hypnotic or intoxicating effects, especially a barbiturate.
Adjective
goofball (not comparable)
(informal) Silly.
Source: Wiktionary