The term 'hard nut to crack' is a metaphor for something that is difficult to solve or understand. The crossword solutions for this term are ACORN (5 letters), PECAN (5 letters), ENIGMA (6 letters), and POSER (5 letters). An ACORN is the nut of an oak tree, while a PECAN is the nut of a hickory tree. An ENIGMA is a puzzle or riddle, and a POSER is a difficult question or problem. All of these words can be used to describe something that is hard to solve or understand.