Other forms: frightened, frightening, frightens
To frighten is to make somebody afraid. Frighten the horses, frightened of the dark, it frightened me.
English is odd here: frighten is the transitive verb and fear is mostly intransitive in feeling, so you frighten somebody but you fear something. Frightened of and afraid of are interchangeable. Note the -en ending, which makes verbs from nouns and adjectives.
1verbfrightenTo make somebody afraid.
“The loud thunder frightened the children, and they hid under the table.”
frighten is a scaring.
Fright plus -en, from Old English fyrhto, fear. The -en ending makes a verb of it.
Middle English
*frightenen
English, today
frighten
From Middle English frightenen, equivalent to fright + -en.
Etymology from Wiktionary, CC BY-SA