unparalleled in the whole country; peerless throughout the land
A yojijukugo (four-character idiom) used to describe someone or something that has no equal within the country. Literary and somewhat formal; not used in everyday conversation.
His swordsmanship was praised as unparalleled in the whole country.
This work of art has peerless value throughout the land.
天下無双 means 'unparalleled under heaven' and is more common. 海内無双 is a rarer variant with a similar meaning but a more literary, classical feel.
A yojijukugo composed of 海内 (within the seas, i.e., the country) and 無双 (unparalleled). The exact historical origin is uncertain, but it follows the pattern of classical Chinese-derived four-character idioms.