In a loyal or faithful manner; with allegiance or devotion.
From the adjective 'allegiant' plus the adverb suffix '-ly' (from Old English 'lice'), which converts adjectives into adverbs describing how an action is done.
This is an adverb built on an already-archaic adjective—'allegiant' → 'allegiantly'—so it's doubly rare in modern English, but it's the perfect word if you want to describe someone acting with old-fashioned, feudal-style loyalty!
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