Compare the Old Church Slavonic късно (kŭsno), оукъснѣти (ukŭsněti, “to be late”) and закъснѣние (zakŭsněnie, “laziness”), all from the adjective късенъ (kŭsenŭ, “late”). kasno and the adjective kasan are compounds from the preposition k (“to”) and the dative of san (“dream, sleep”). It was originally used to indicate someone who comes too late because he/she failed to wake on time. Possible cognates include Bulgarian късно (kǎsno), Russian косный (kosnyj, “inert, lifeless”), Lithuanian kušlas (“weak, tired”), Latvian kusu (“tired”).
kȁsno (Cyrillic spelling ка̏сно)