at that very moment; just then
Used to emphasize that something happened exactly at a particular time, often coinciding with another event. Typically found in written or formal contexts.
Just then, a loud thunderclap sounded outside.
As it happened, that day was his birthday.
Compound of 折 (おり, 'time, occasion') + しも (emphatic particle). The particle しも adds emphasis, yielding the meaning 'exactly at that time'.