expression
in case of emergency; should danger arise
A formal, somewhat literary yojijukugo used to express preparedness for a sudden crisis or emergency. Often appears in written or set-phrase contexts.
一旦緩急の際には、このマニュアルに従って行動してください。
In case of emergency, please act according to this manual.
彼は一旦緩急に備えて、常に非常持ち出し袋を用意している。
He always keeps an emergency kit ready, just in case.
A yojijukugo combining 一旦 (once, in case) and 緩急 (slowness and urgency, i.e., emergency). The exact historical derivation is uncertain, but it is conventionally used to mean 'in case of emergency'.