From LookUp:
mid 16th century: perhaps a variant of dulled, past participle of dull
Noun
Similar Words: fool nincompoop clown simpleton idiot ninny dope dimwit chump goon dumbo dummy halfwit nitwit dum-dum loon jackass cretin imbecile jerk nerd fathead blockhead numbskull dunderhead dunce dipstick bonehead chucklehead clod goop knucklehead lamebrain pea-brain pudding-head thickhead wooden-head pinhead airhead birdbrain dumb-bell donkey stupe noodle nit twit numpty clot plonker berk prat pillock wally git wazzock divvy nerk twerp charlie mug muppet nyaff balloon sumph gowk gobdaw schmuck bozo boob lamer turkey schlepper chowderhead dumbhead dumbass goofball goof goofus galoot dork lummox klutz putz schlemiel sap meatball gink cluck clunk ding-dong dingbat wiener weeny dip simp spud coot palooka poop squarehead yo-yo dingleberry drongo dill alec galah nong bogan poon boofhead mompara arse arsehole knobhead twat wanker ass asshole asshat tomfool muttonhead noddy clodpole loggerhead spoony mooncalf