This is the 12729th most frequent Russian word.
крапивой
"Крапивой" is the instrumental case of "крапива," meaning "with nettle" or "by means of nettle."
In this sentence, 'крапивой' is used in the instrumental case, describing the instrument by which the action occurred.
Он обжёгся крапивой, гуляя по лесу.
He got stung by nettle while walking in the forest.
Here, 'крапивой' is in the instrumental case, describing the covering material of the field.
Прохожие заметили поле, покрытое крапивой.
The passersby noticed a field covered with nettle.
This sentence demonstrates 'крапиву' in the accusative case as it is the direct object of the action, not 'крапивой.'