по-чёрному

Etymology
Adverbial form of, compare the usage in 🇨🇬, as in

Adverb

 * 1) (of ovens) without a chimney when a smoke goes directly into the house
 * 2)  illegally, without a permit; secretly
 * 3)  severely; cruelly; mercilessly
 * 1)  severely; cruelly; mercilessly