1. in a manner that constitutes or involves treason; betraying one’s country or sovereign
Usage: formal; legal

Examples

• The general was accused of acting treasonably by sharing military secrets with the enemy.
• Documents showed he had treasonably conspired against the government.
• The court determined that his actions were treasonably motivated.
• She was charged with treasonably aiding foreign agents.
• The spy had treasonably violated his oath of allegiance.
• His treasonably disloyal conduct shocked his former colleagues.