Synonyms for Scatterbrained:

adj.

capricious (adjective)
fly-by-night, random, volatile, fluctuating, changeable, fitful, flighty, fanciful, frivolous, capricious, erratic, arbitrary, indecisive, impulsive, wayward, fleeting, fickle, sporadic, whimsical, Flitting, vacillating.
not thinking clearly (adjective)
flighty, dizzy, forgetful, harebrained, empty-headed, giddy, birdbrained, silly, featherbrained, irresponsible, illogical, irrational, frivolous.

n.

mental block, slip through the net/cracks, block, something goes in one ear and out the other, senior moment, forgetful, oblivion, Absent-minded, get/become lost (in something). giddy, frothy, ability, harebrained, empty-headed, dizzy, featherbrained, silly, birdbrained, lighthearted, gaga. scatterbrained (noun)
scatty, rattlebrained, rattlepated, foolish, irresponsible, flighty, head-in-the-clouds.
silly (noun)
illogical, irrational, giddy.

Other synonyms:

gaga, birdbrained, featherbrained. empty-headed. harebrained. lighthearted, silly, frothy. giddy. dizzy. abstracted
Absent-minded.
Other relevant words:
frothy, birdbrained, irresponsible, giddy, gaga, irrational, foolish, dizzy, lighthearted, Absent-minded, featherbrained, silly, rattlebrained, illogical, empty-headed, harebrained, scatty, rattlepated, forgetful, head-in-the-clouds.