foolish, silly; mindless ...1823
from hollow (empty, vacant) + -headed
FIRST DOCUMENTED USE
1823 - see EXAMPLE below
"...Young R. Was ever man plagu'd with such a hollow-headed ninny-hammer.
Frank. [Aside] Maybe, that be better han a hollow-hearted one!..."
From: The British Theatre; Or, A Collection of Plays,: Which are Acted at the Theatres Royal, Drury-Lane, Covent-Garden, and Haymarket ...
- Mrs. Inchbald