Complain ad nauseam is a phrase used to describe someone who complains excessively and without end. It is derived from the Latin phrase 'ad nauseam, ' which literally translates to 'to sickness.' The term is often used to describe someone who complains about the same thing over and over again. In the crossword, the solution for the term Complain ad Nauseam is WHINE, which is an appropriate word to describe someone who complains excessively.