The adjective inane means an action or behavior that is silly or stupid.

Synonyms are absurd, frivolous, laughable, or futile.

The word origins from Latin (1660s) inanis meaning “empty, void”, or else a back-formation from inanity (q.v.). Its sense of “silly, empty-headed” is first used in 1819.

His little sister does inane things that brightens up his day.

He tends to act inane around his friends.

You will never see that strict professor act inane.