ゆさい【夜さい・夜去い・夜去】 : yusai | define meaning

Cognate to Fukui よさり yosari; Toyama よさる yosaru; Kumamoto よさり yosari and よさる yosaru "evening, night"; and Kikai ゆさい yusai "tonight". Related to Kagoshima ゆさいもと yusaimoto and よさいもと yosaimoto "dusk, evening".

Per the 日本国語大辞典 Nihon Kokugo Daijiten: Historically attested in Japanese under the forms ようさり yousari, よさり yosari, よさ yosa and ゆうさり yuusari "evening, night, tonight, in the middle of the night". The form よさり yosari ultimately comes from  よ【夜】 yo "night, evening" and さり【去り】 sari "coming (or going) off in the distance"; while the form ゆうさり yuusari comes from ゆう【夕】 yuu "evening" and さり【去り】 sari.