Fatuous (adjective)

1. Self-satisfied and foolish.
2. Lacking intelligence or common sense.

Origin:

Late 16th century (in the sense "empty, vain"): from latin fatuus "foolish", from fatus "spoken, said".

Examples:

1. The fatuous claims made by the politician were quickly debunked by fact-checkers.
2. His fatuous grin as he took credit for someone else's work made everyone in the office roll their eyes.
3. The fatuous argument that the earth is flat is not supported by any scientific evidence.
4. She gave him a fatuous smile and told him everything was fine, but inside her heart was breaking.
5. He was so fatuous that he didn't realize the prank had been played on him.