treacherous

Concise Oxford English Dictionary © 2008 Oxford University Press:
treacherous /ˈtrɛtʃ(ə)rəs/

▶adjective

• 1 guilty of or involving betrayal or deception.

• 2 (of ground, water, conditions, etc.) having hidden or unpredictable dangers.

– derivatives
treacherously adverb,
treacherousness noun.

– origin ME: from OFr. trecherous, from trecheor ‘a cheat’, from trechier ‘to cheat’.