na-adjective, noun which may take the genitive case particle 'no', noun
peerless; unrivaled; invincible
Describes someone or something so strong or excellent that nothing in the world can match it. Often used in a dramatic or emphatic way, like a title or boast.
He boasts of being peerless in kendo.
This team showed invincible strength this year.
Very similar in meaning; 天下無双 emphasizes being one of a kind, while 天下無敵 focuses on being unbeatable.
Also means peerless, but 天下無比 is less common and slightly more literary.
A yojijukugo composed of 天下 (the world, under heaven) and 無敵 (no enemy, unmatched). The exact historical origin is uncertain, but it follows a common pattern of four-character idioms expressing supreme excellence.