An older or alternative spelling of 'frightener'; something that causes fright or fear.
From 'fright' (noun and verb) plus the agent suffix '-er,' representing an older spelling pattern before 'frighten' became the standard verb form in English.
This older word 'frighter' shows how English spellings evolved—we used to see 'frighter' in older texts, but it was gradually replaced by 'frighten' as the standard verb form.
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