the other night; a few nights ago
Refers to a recent night, typically within the past few days. Often used in narrative or explanatory contexts.
I had a strange dream the other night.
先夜の出来事を詳しく話してくれた。
He told me in detail about the events of a few nights ago.
Compound of 先 (せん, 'previous') and 夜 (や, 'night'). The exact historical derivation is uncertain, but the meaning is transparent from its components.