A proverb meaning 'even after victory, tighten your helmet strap' — i.e., don't let your guard down after success.
試験に合格したが、先生は『勝って兜の緒を締めよ』と言ってさらに勉強を続けるよう促した。
Although I passed the exam, the teacher told me 'tighten your helmet strap after victory,' urging me to keep studying.
契約を取れたが、社長は勝って兜の緒を締めよと注意を促した。
They got the contract, but the CEO warned them not to relax after victory.