Answer
"The Pittsburgh Windmill" nickname perfectly encapsulates Harry Greb's relentless fighting style and aggressive nature in the boxing ring. It alludes to the constant motion and whirlwind of punches he unleashed upon his opponents, leaving them bewildered and overwhelmed. Greb's unceasing attacks resembled a windmill's relentless spinning, making him a formidable and dreaded adversary during his boxing career in the early 20th century.