A four-character idiom referring (figuratively) to an exceptionally rare and outstanding person — literally 'a person unmatched in the country.'
あの研究者は国士無双と言われるほどの業績を残した。
That researcher left achievements that led people to call him a rare talent.
文学界で国士無双と評される作家がいる。
There is a writer in the literary world who is praised as a rare genius.
A special mahjong winning hand (Thirteen Orphans): one of each terminal and honor tile plus one extra copy of any of them.
彼は最後のツモで国士無双を和了して場を沸かせた。
He won with a Kokushi Musou on the final draw and excited everyone at the table.
国士無双は揃う確率が非常に低く、出たら大きな得点になる。
Kokushi Musou has a very low probability of occurring, and yields a high score when it does.
その配牌で国士無双を狙うのは現実的ではないが、夢を見るのは自由だ。
It's not realistic to aim for Kokushi Musou with that hand, but it's free to dream.