In an alacrious manner; with speed, eagerness, and liveliness; promptly and animatedly.
From alacrious plus -ly (adverbial suffix). This creates the adverb form from the adjective, following standard English morphology for creating manner adverbs.
This word is practically invisible in modern English because everyone says 'alacrity' instead—but 'he responded alacriously' is technically perfect grammar and sounds wonderfully archaic!
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