Almost always used adverbially as いたずらに, meaning 'in vain', 'to no purpose', or 'idly'. The standalone adjectival use is rare.
I ended up spending time idly.
It's no use worrying in vain.
useless fellow; down-and-out person · dead person
nonsense; gibberish; meaningless words
Kana spelling is standard for this adjectival noun; the kanji forms 徒 and 徒ら are rare and literary.
Rare kanji spelling with okurigana; literary or archaic feel.
Rare kanji spelling; may be confused with other readings such as と.
The origin is uncertain. The kanji 徒 carries meanings of 'futility' or 'in vain', but the word is now usually written in kana.