vital part (of the body); tender spot; weak point; vitals
key point; essential point; crux (of a problem); heart (of a matter)
male crotch (as a target in fighting)
《go (game)》vital point; critical place to make a move
急所を一撃で仕留めた。
He struck the vital point in a single blow.
議論の急所を見抜いた解答だった。
It was an answer that got to the heart of the argument.
計画の急所を見落としたのが失敗の原因だった。
Overlooking the critical point in the plan was the cause of the failure.