This phrase describes a crossword puzzle exhibiting characteristics of extreme difficulty, complexity, and intricacy, potentially requiring specialized knowledge or advanced solving techniques. It suggests a puzzle designed to challenge even the most experienced solvers, much like a demanding workout might challenge a physically fit individual. An example might be a crossword with obscure vocabulary, complex wordplay, or an unconventional grid structure.
Highly challenging crossword puzzles offer significant benefits for cognitive function. They stimulate problem-solving skills, enhance vocabulary and knowledge retention, and improve memory and focus. Historically, complex puzzles have served as a form of intellectual entertainment and a measure of mental acuity. The emergence of themed crosswords and increasingly sophisticated construction techniques has further contributed to the development of puzzles that provide a substantial cognitive workout.