This is the 13765th most frequent Russian word.
нечистой
"Unclean" or "impure."
The adjective 'нечистой' describes the stream as impure or contaminated based on visual or contextual judgment.
Этот ручей кажется нечистой, поэтому не пей из него.
This stream seems impure, so don't drink from it.
The phrase 'нечистой силы' refers to supernatural or malevolent forces, a common usage in folklore and mystery contexts.
В деревне ходят слухи о нечистой силе, живущей в лесу.
In the village, there are rumors about an unclean force living in the forest.
Here, 'нечистой' characterizes the yard as dirty or unkempt, emphasizing physical untidiness.