(n.) An extremely difficult and challenging problem or deadlock\.

Chinese puzzle Rube Goldberg contraption can of worms complex crux dilemma enigma jungle knot knotty point labyrinth maze meander mesh mess node nodus nonplus oxymoron paradox perplex perplexity pons asinorum poser puzzle quandary ravel snafu snake pit snarl tangle tangled skein teaser vexed question webwork wheels within wheels wilderness