'frightener' originates from English, specifically formed from the verb 'frighten' (itself from the noun 'fright') with the agentive suffix '-er' attached to create a noun meaning 'one that frightens'.

'fright' changed from Old English 'fyrhtu' (or related forms) into Middle English 'fright(e)'; the verb-forming suffix '-en' produced 'frighten', and adding the agentive suffix '-er' produced 'frightener'.

Initially it referred to the concept of 'sudden fear' (associated with 'fright'), but over time derivatives came to denote 'a person or thing that causes fear' — hence 'frightener'.