Wiktionary
a. foolish
WordNet
adj. lacking sense or discretion; "his rattlebrained crackpot ideas"; "how rattlepated I am! I've forgotten what I came for"- Glenway Westcott [syn: rattlepated, scatterbrained, scatty]
Usage examples of "rattlebrained".
She was a rattlebrained fool who would undoubtedly giggle and take it as a great joke when the Cauldron-Born came for him.
He discovered that he was actually aflame with impatience and excitement, that he was filled with a rattlebrained ardor of the sort he usually managed to avoid.
Susie had a certain rattlebrained effervescence that was amusing and, after a time, one began to suspect it was something she was born with, just as she had been born with the blonde hair, and that, underneath, there was quite a bit of hard-headed sense.
Savage devoted considerable thought to Susie during the southward trip, without reaching much of a conclusion as to whether she was rattlebrained or had keen intelligence.