The superlative form of 'scary,' meaning the most frightening or causing the most fear.
From 'scary' (from 'scare,' possibly from Old Norse 'skirra' meaning 'to run away'), plus the superlative suffix '-est.' The 'sc-' cluster suggests Germanic origins related to flight or fright.
Interestingly, we say 'scariest' but our brains react more strongly to things that are uncertain or seem 'unpredictable scary'—horror movies know this, which is why the scariest moments are often what you don't see rather than what you do.
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