I've never heard this one used. You more often here birdbrain, nitwit, or ditz.

52 minutes ago, JamesSavik said:

I've heard rattlebrain described a little differently.

Sometimes when you get older and your brain gets full, it doesn't always come out in the right order, or when you need it to. It's a bit like the junk drawer in your kitchen, which has all sorts of useful miscellaneous stuff, if only you could remember what was in there. Clothespins to seal potato chip bags? ✔️ Small Hammer? ✔️ Screwdriver? ✔️ Toothpicks? ✔️ Matches? ✔️ 8" Lighter for the grill? ✔️

Now, if only my silly rattlebrain could remember what was in there, I could probably run the whole house from that drawer.

All of my drawers are junk drawers. I get terrified whenever I go to someone's house and I'm confronted with orderly drawers. It's so unnatural.

56 minutes ago, Bill W said:

I've never heard the word rattlebrained before.  My grandparents and other older relatives used the word addle-brained or addle-headed instead, which basically means the same thing as rattlebrained.  

Or addlepated. 

I'm sure I've heard it in conjunction with a scare! 

After the mummy jumped out from behind the wall, it left me rattlebrained for a bit.