Patsy Gallacher earned the nickname "The Mighty Atom" due to his small stature combined with his incredible skill and tenacity on the football pitch. Standing at just 5 feet 5 inches tall, Gallacher was known for his agility, speed, and ability to outmaneuver larger opponents. His remarkable talent and performances, particularly during his time at Celtic and in the Scottish national team in the 1920s and 30s, made a significant impact in Scottish football, leading to the affectionate moniker that highlighted both his physical size and his larger-than-life presence in the game.

For more detailed insights on his career and impact, you can refer to sources like Scottish Football Association.