Describes a personality trait of being ready to confront or challenge others; often used in contexts of spirited defiance or a fighting attitude.
He is very aggressive and quick to pick a fight.
We need a player with a fighting spirit on the team.
Compound of 向こう (mukō, 'opposite side, other party') + っ気 (kki, a suffix indicating a tendency or temperament). The suffix っ気 derives from 気 (ki, 'spirit, disposition').