na-adjective, noun which may take the genitive case particle 'no', noun
extremely difficult; next to impossible
A relatively formal word for something so hard that it is almost impossible. Common in set-like phrases such as 至難の業 and 至難を極める.
この条件で全員を納得させるのは至難だ。
Under these conditions, convincing everyone is extremely difficult.
彼の記録を破るのは至難の業だ。
Breaking his record is a nearly impossible task.
A Sino-Japanese compound written with 至, associated with reaching an utmost degree, and 難, difficulty. The spelling transparently conveys 'utmost difficulty.'