One who hastes or hurries; a person who moves or acts quickly; someone inclined to haste.
From 'haste' (noun) + suffix '-er' (agent noun). This is an alternative agent noun formation from the noun 'haste' rather than from the verb 'hasten.'
In older English, you could add '-er' to nouns as well as verbs to make agent nouns—'a haste-er' (one who does haste) was more common before 'hastener' became the standard form, showing how word preferences shift over time.
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