adverb taking the 'to' particle, taru-adjective
smiling broadly; beaming
Literary adverb describing a broad, gentle smile. Used with と (e.g., 莞爾として) or in the form 莞爾たる. Rare in everyday speech; mainly found in written narratives.
He left the place with a broad smile.
With a beaming smile, she nodded.
The kanji 莞 (かん) means 'smiling' or 'happy' and 爾 (じ) is a classical auxiliary character. The word is of Chinese origin (cf. Mandarin wǎn'ěr) and entered Japanese as a literary expression.