Green Boots is a nickname. I'd say it's the nickname of a person, but I'd be wrong.
On the north side of Mount Everest, on the main trail a short way down from the summit, there is a shelter - a rock formation with an overhang beneath which it is possible to huddle out of the wind. In that shelter lies a man huddling out of the cold, his arms wrapped tightly around his midsection. His feet rest outside the shelter, on the trail itself. They are clad in neon green climbing boots.
The man is dead. He died in 1996, in the same storm that is described by Jon Krakauer in the book Into Thin Air. He, along with three fellow members of the Indo-Tibetan Border Police, were attempting the summit on May 10, 1996. Their commander, suffering from frostbite, was lagging behind badly; although he ordered his men to stop, they either did not or would not hear him, so he returned to camp that morning alone. He contacted his men from there via radio. They pleaded with him to be allowed to summit, as they were extremely close, and were perhaps suffering from 'summit fever' - the desire to complete the possibly once-in-a-lifetime journey before returning, despite any obstacles that would prevent someone making the decision dispassionately from continuing. They continued on. They radioed back to indicate they had reached the summit. Although this is disputed, and it's possible they unknowingly stopped 150 meters short due to poor visibility, they are credited with reaching it.
On the way back down, however, the blizzard that Krakauer describes broke. Radio contact was lost during their summit report. There are varying reports from other climbers of having encountered them, and there is a controversy over whether a Japanese team that reported seeing them should have assisted them - but none of the three made it back. Tsewang, 28 years old, evidently made it as far as the shelter before succumbing.
What is probably the most disturbing thing about his corpse is that it looks like he lay down to rest a few hours earlier, even in pictures taken in 2014. The extreme conditions of the mountain have mummified him, although none of his skin is visible, and the shadowed overhang has even protected his gear from sun bleaching. Ten years later another climber, named David Sharp, succumbed in the same spot. His body was found hunched next to Green Boots in the shelter. For those of us not used to seeing the dead on a regular basis, it's almost inconceivable that what to us at sea level is a 'public way' has a prominently visible corpse along it for so long. But for nearly twenty years, climbers have stepped over and around his green boots on their way to and from the summit. The logistics involved mean that bodies on the mountain are rarely removed. Ask anyone who knows anything about Everest, and they'll know about Green Boots. It's not the nickname of a man, but the nickname of his body and the landmark it has become. His corpse is not alone, either; as RedOmega points out to me, there is a whole area of the northeast summit (I think) called 'Rainbow Valley' - so named for the bright colors of the arctic wear that the various corpses there are wearing. Since being first climbed, Everest has claimed over 215 lives, and over 150 of those casualties were never recovered from the mountain. Some of them are famous landmarks to climbers, not just Green Boots.
His name was Tsewang Paljor. His comrades were Tsewang Smanla and Dorje Morup. Their commander, Harbhajan Singh, survived due to turning back to Camp VI after suffering frostbite and serves in the ITBP today.
(Due to the number of gaily colored corpses within, submitted for the Horrorquest!)
Information taken from the BBC, Wikipedia, and Smithsonian Magazine.