1. in a way that involves betrayal or disloyalty to someone who trusts you
2. in a dangerous or deceptive way that cannot be trusted or relied upon

Examples

• The spy treacherously revealed state secrets to enemy agents.
• He treacherously abandoned his allies when they needed him most.
• The ice looked solid but treacherously gave way under their weight.
• The mountain path treacherously narrowed without warning.
• She treacherously spread rumors about her closest friend.
• The calm waters treacherously concealed dangerous currents below.