taru-adjective, adverb taking the 'to' particle
remaining calm after success; not getting carried away by achievement
Archaic literary expression describing a composed, unassuming attitude after a great success. Used as a たる adjective or adverb with と.
Even after a great victory, he remained calm and unassuming.
得意満面 means 'beaming with pride' or 'looking very pleased with oneself', the opposite of 得意淡然's restrained composure.
A yojijukugo (four-character idiom) combining 得意 (success, pride) and 淡然 (calm, composed). The exact historical origin is uncertain, but it reflects a classical ideal of modesty in achievement.