WordNet (r) 3.0 (2006):

goofball
    n 1: a man who is a stupid incompetent fool [syn: fathead,
         goof, goofball, bozo, jackass, goose, cuckoo,
         twat, zany]
    2: a person who amuses others by ridiculous behavior [syn:
       clown, buffoon, goof, goofball, merry andrew]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "goofball":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey Finn, Nembutal, Nembutal pill, Seconal, Seconal pill,
   Tuinal, Tuinal pill, alcohol, amobarbital sodium, analgesic,
   anodyne, barb, barbiturate, barbiturate pill, black stuff, blue,
   blue angel, blue devil, blue heaven, blue velvet, butterfingers,
   calmative, chloral hydrate, clumsy, codeine, codeine cough syrup,
   depressant, depressor, dolly, downer, dub, duffer, foozler,
   foul-up, fumble-fist, goof, goofer, hard stuff, heroin, hick, hop,
   horse, hypnotic, junk, knockout drops, laudanum, liquor, lotus,
   lummox, lump, meperidine, methadone, morphia, morphine, muff,
   muffer, narcotic, opiate, opium, pacifier, pain killer, paregoric,
   pen yan, phenobarbital, phenobarbital sodium, purple heart,
   quietener, rainbow, red, rube, scag, secobarbital sodium, sedative,
   shit, sleep-inducer, sleeper, sleeping draught, sleeping pill,
   slob, smack, sodium thiopental, somnifacient, soother,
   soothing syrup, soporific, stumblebum, stumblebunny, tar,
   tranquilizer, turps, white stuff, yellow, yellow jacket