Dangerous and likely to fail or cause harm unexpectedly; or disloyal and betraying trust.
From Old French 'trechier' meaning 'to deceive,' derived from Latin 'trichare.' The '-ous' suffix adds the meaning of 'full of.' It originally referred to people who betrayed others, then expanded to describe dangerous situations that 'betray' your expectations.
Mountain climbers use 'treacherous' for icy slopes because mountains literally betray you—they *look* solid but hide deadly danger. This is why the same word works for both a fake friend and a slippery cliff: both break a promise of safety.
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