To make fierce or more fierce; to intensify in ferocity or intensity.
Formed from 'fierce' (adjective) with the common English verb-forming suffix -en (from Old English -nian), which creates verbs meaning 'to make' or 'to become,' as in 'darken,' 'lengthen,' 'weaken.'
The -en suffix is one of English's most ancient and productive features—we can take almost any adjective and add -en to create a verb. This flexibility helped English survive and evolve without constantly borrowing new verbs from other languages.
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