Looking dark, dangerous, or unfriendly in a way that makes people want to stay away or feel scared.
From 'forbid' (Old English 'forbeodan', to prohibit), the '-ing' form developed to describe something that appears to warn people away.
Gothic architecture's forbidding castles were partly designed that way intentionally—steep walls and narrow windows didn't just defend, they psychologically intimidated potential attackers before battles even started.
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