2. Referring to a solid mass in a tree which is the result of the growth of a branch, creating a circular grain in the wood, as opposed to a straight grain when cut: The office was decorated with a panel of knotty pine wood.
A supervisor's secretary told her fellow worker, "Mr. Smith doesn't want to be disturbed. He says he's working on a naughty problem."
Her colleague replied with, "Hmmm, that sounds just like Mr. Smith."
Overhearing the conversation, the supervisor opened his door and responded with: "I said I was working on a knotty problem, not a naughty problem!"
The principal was faced with a knotty problem; that is, what to do about the two naughty youths who used spray paint on the knotty pine-board partition.