Since some are apparently unfamiliar with what this term means, and apparently think it means "velly, velly good", I reference Shaw:
A "double-superior" condition occurs when one fighter has both significantly higher T/W [Thrust-weight ratio] and lower wing loading than its opponent. Obviously the unlucky adversary in this situation is "double-inferior".
--Robert Shaw, Fighter Combat, page 183.
Thank you, that is all.