suru verb (special), transitive verb
lose; miss; forget; be excessive
A somewhat formal or literary transitive suru verb. The meaning 'be excessive' is rare and mostly seen in fixed expressions like 度を失する.
彼は冷静さを失して怒鳴り始めた。
He lost his composure and started shouting.
大事な書類を失してしまった。
I lost an important document.
度を失した発言は控えるべきだ。
You should refrain from remarks that go too far.