In a manner that is beautiful, attractive, or graceful; with beauty or aesthetic appeal.
From 'beauteous' (adjective meaning beautiful, from Middle English and Old French), plus the '-ly' adverbial suffix that converts adjectives to adverbs describing manner or quality.
While 'beautifully' is more common today, 'beauteously' shows up in older poetry and literature—it's got a more romantic, almost Shakespearean flair to it!
Complete word intelligence in one call. Free tier — 50 lookups/day.