taru-adjective, adverb taking the 'to' particle
Literary or formal bound form, typically appearing in compounds or as a taru-adjective/adverb with たる/と. Describes physical softness or metaphorical gentleness/pliability.
彼の態度は軟にして剛を制す。
His attitude is soft yet overcomes the hard.
春の風が軟と吹く。
The spring breeze blows gently.