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.
He is excellent at math.
With an exultant smile, she received the certificate.
Compound of 大 (だい, 'big, great') and 得意 (とくい, 'strong point, pride'). The exact historical derivation is uncertain, but the word follows a common pattern of intensifying a quality with 大.