Synonyms for Rattlebrained:

adj.

all (adjective)
rattlepated, scatty, scatterbrained.
bird-brained (adjective)
rattlepated, silly, scatty.
ditsy (adjective)
scatterbrained.
harebrained (adjective)
foolish.

n.

giddy (noun)
silly, irrational.
rattlebrained (noun)
foolish, scatterbrained, scatty, rattlepated.

Other synonyms:

Other relevant words:
silly, irrational.