Invincible; peerless — without equal or rival, often used to boast that someone or something is unmatched.
彼は卓球部で天下無敵の実力を持っていると言われている。
He is said to have invincible skill in the table tennis club.
このカメラは操作が簡単で、初心者には天下無敵の選択だと思う。
This camera is easy to use; I think it's an unbeatable choice for beginners.
漫画の主人公が『天下無敵』と自称する場面はよくあるギャグだ。
The scene where a manga protagonist calls himself 'invincible' is a common gag.