to not be loyal to your country or a person, often by doing something harmful such as helping their enemies
The man betrayed his country when he gave away national secrets.
John felt deeply hurt when he discovered that his closest friend had betrayed his trust by sharing his personal secrets with others.
The spy had to be extremely cautious not to betray his true identity to the enemy agents during the covert operation.
Her actions appeared to betray her true feelings, despite her attempts to conceal them.
