1Frantic or desperate — doing something with all one's might or under great pressure, showing extreme urgency.
締め切りに間に合わせようと必死で作業した。
I worked frantically to make the deadline.
母は必死になって迷子の子供を探した。
The mother desperately searched for her lost child.
彼は必死の形相で試験に取り組んでいた。
He tackled the exam with a desperate expression.
2The literal meaning of 'certain death' — an unavoidable death, used as a noun in a somewhat literary or formal register.
敵に包囲されれば必死は免れないと悟った。
He realized that if surrounded by the enemy, death would be unavoidable.
3In shogi, a technical term for a position in which the opponent cannot avoid being checkmated in the coming moves (brinkmate).
この局面はすでに必死がかかっている。
This position is already in brinkmate — checkmate is unavoidable.