A knotty board.
A knotty problem.
Primarily, it was Jerome Shipton's severed climbing rope that remained a knotty question that wouldn't go away.
Their obscure and knotty language only serves to give peculiar brilliancy to the not uncommon passages of noble perspicacity.
He was perhaps still more of a lawyer: his delight was in knotty points of the law, and he knew the Assises better than any of his subjects.