From лони́ (loní) + -сь (-sʹ). Compare днесь (dnesʹ), вчерась (včerasʹ), летось (letosʹ). IPA(key): [ɫɐˈnʲisʲ] <span class="searchmatch">лони́сь</span> • (lonísʹ) (dialectal) last year...
terms referring to time) лони́ (loní, “year”) (dialectal) + -сь (-sʹ) → <span class="searchmatch">лони́сь</span> (lonísʹ, “last year”) (dialectal) вчера́ (včerá, “yesterday”) + -сь (-sʹ)...