Causing fright; frightening or scary in appearance or nature; inspiring fear or terror.
From 'fright' plus the suffix '-some,' which creates adjectives meaning 'characterized by' or 'tending to' (like 'lonesome' or 'awesome').
The suffix '-some' creates adjectives like 'frightsome' and 'fearsome' that sound quite dramatic and old-fashioned—they'd fit perfectly in Gothic novels but sound theatrical in everyday speech.
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