na-adjective, noun which may take the genitive case particle 'no', noun
peerless; unequaled; unrivaled
A formal, somewhat literary set phrase meaning 'unmatched under heaven'. Often used in historical or dramatic contexts to describe supreme excellence. Compare 天下無双 (てんかむそう) and 天下無敵 (てんかむてき), which are more common.
He was praised as a peerless master swordsman.
This castle is said to be of unrivaled beauty.
A Sino-Japanese compound of 天下 (てんか, 'under heaven, the whole world') and 無比 (むひ, 'unparalleled, matchless'). The phrase follows classical Chinese patterns and entered Japanese as a literary set expression.