A knotty problem is a difficult issue or challenge that requires a great deal of thought and effort to solve. The crossword solutions for this term are Nodus (5 letters), POSER (5 letters), and TESTER (6 letters). Nodus is Latin for 'KNOT' and can refer to a complicated situation or problem. POSER is a French word meaning 'to pose a question' and can be used to describe a difficult problem or puzzle. TESTER is a noun meaning 'one who tests' and can be used to refer to someone who is trying to solve a difficult problem.