What does the nickname "Leatherneck" refer to?

The nickname "Leatherneck" specifically refers to the leather collars that were part of the Marine Corps uniform in the 19th century. These collars were designed to protect Marines' necks from sword slashes and were a distinctive feature of their uniforms during that time. The use of leather not only served a practical purpose but also symbolized resilience and toughness, traits that are associated with being a Marine.

The other options do not accurately capture the historical significance of the term. While the physical toughness of Marines is certainly a characteristic attributed to them, it does not explain the origin of the nickname. Similarly, the fabric of uniforms and military leather boots are not directly connected to the term "Leatherneck." The correct understanding of this nickname highlights the Marine Corps' rich history and connection to their heritage.