From Wiktionary, the free dictionary
From сён- (sön-) + -юк (-ük). Compare Azerbaijani and Turkish sönük.
сёнюк • (sönük)
- (of light or colors) faded, dim, dull, extinct
- (of mood) gloomy, depressed
- Бамматов Б.Г., editor (2013), “сёнюк”, in Кумыкско-русский словарь [Kumyk–Russian dictionary], Makhachkala: ИЯЛИ ДНЦ РАН