noun, noun which may take the genitive case particle 'no'
the best in the land; unrivaled in the country
Emphatic, somewhat literary expression meaning 'the very best in the whole country'. Often used in historical or dramatic contexts, or in set phrases like 天下第一の～.
彼は天下第一の剣豪と称えられた。
He was praised as the greatest swordsman in the land.
この城は天下第一の堅固さを誇る。
This castle boasts the strongest defenses in the whole country.
Compound of 天下 (tenka, 'the whole country; the world') and 第一 (daiichi, 'number one; the best'). The exact historical derivation is uncertain, but the phrase follows classical Chinese-influenced word formation.