Final answer:
The nickname given to individuals who participated illegally in the Oklahoma land rushes of the 1880-90s is 'Sooners.' Sooners entered the territory before the official start of the land rush to claim the best land spots.
Step-by-step explanation:
The nickname given to individuals who participated illegally in the land rushes of the 1880-90s that initially opened the Oklahoma Indian Territory to non-native settlement is 'Sooners.'
The term 'Sooners' refers to those who illegally entered the territory before the official start of the land rush to stake their land claims. These individuals often snuck into the territory ahead of time and marked their spots, gaining an unfair advantage over others.
Sooners were called so because they entered the territory sooner than they were allowed to and sought to gain an advantage in claiming the best land.