1. not based on valid reasoning or good judgment; faulty or unreliable
2. not physically strong, stable, or in good condition
3. not mentally or emotionally healthy

Examples

• The lawyer’s argument was unsound and easily refuted.
• His financial advice proved to be completely unsound.
• The old bridge was declared unsound and closed to traffic.
• The building’s foundation was unsound after the earthquake.
• The committee rejected the proposal as economically unsound.
• She was declared mentally unsound and unfit to stand trial.
• The investment strategy seemed unsound from the beginning.