na-adjective, noun which may take the genitive case particle 'no'
excellent; very good; exultant; elated
Describes something one is extremely proud of or very good at, or a state of great satisfaction and delight. Often used for a person's strong suit or a triumphant mood.
彼は数学が大得意だ。
He is excellent at math.
大得意の笑顔で賞状を受け取った。
With an exultant smile, she received the certificate.