A foolish, unintelligent, or dim-witted person. An allusion to the small size of the fictional cartoon characters known as Smurfs. Her new boyfriend certainly is attractive, but he's a bit of a smurfbrain, don't you think? Sorry, I've been a total smurfbrain all day. I didn't get a whole lot of sleep last night.
n. a simple-minded person. (A smurf is an innocent little cartoon character.) You can be such a smurfbrain!