Unsound (adjective)

1. Physically or mentally unhealthy.
2. Not based on good reasoning or evidence.
3. Not firm or secure.

Origin:

From the old english word 'unsund' meaning 'not healthy or safe'.

Examples:

1. The horse was deemed unsound and was unable to race.
2. The argument was unsound and lacked credibility.
3. The building was deemed unsound and had to be torn down.
4. Her mental state was unsound and she needed professional help.
5. The financial plan was unsound and had to be revised.