Of unsound mind
Definition 1 of 1
Pro Tip 1/3Legal Consequences
Used to determine if someone is responsible for their actions, often affecting legal outcomes. 
He was absolved as he was of unsound mind during the incident.
Pro Tip 2/3Sensitive Usage
Considered disrespectful when used casually due to the association with mental health conditions. 
The term 'of unsound mind' should be used carefully to avoid offending people.
Pro Tip 3/3Context Awareness
Most appropriate in formal and legal contexts, not in everyday conversation. 
The contract was voided because one party was of unsound mind.