Research conducted by a Senior Linguist of the Smithsonian institute.
The earliest use of the word "Red Skin" in print was in July 22nd, 1815 newspaper quoting a native american chief stating "I have never injured you, and innocence can feel no fear. I turn to all red skins and white skins, and challenge an accusation against me."
The earliest discovered reference to the word in history was from 1769, when a chief named Mosquito "And if any redskins do you harm, I shall be able to look out for you even at the peril of my life."
The earliest public reference of the word used in English was in 1812 by James Madison and a number of tribal leaders who made statements such as "I know the manners of the whites and of the red skins" and "I am a red-skin, but what I say is the truth, and notwithstanding I came a long way, I am content, but wish to return from there."
Every dictionary classifies it as a usually offensive term.
But even your reliance on the dictionary, it still does not follow what you claimed which was that it IS a racial slur...not that it USUALLY is. And I'd argue that the dictionary is hardly correct, since the words is used FAR more in society as a reference to a football team than it is as a direct reference to native americans, let alone as a slur. However, a word's relation to a team isn't part of a dictionary definition typically....which is why the ethnic slur "yankee" doesn't make mention of a baseball team.
SOME native americans have been.
And they can all go suck a duck. Peter King and his MMQB no longer receives any clicks from me. They're more than happy to indulge in their protest...their protest proves nothing about the name other than their displeasure with it.
I'm not advocating forcing the Redskins to change their name. But they do deserve all of the PR backlash and boycotts that result from keeping the name. They have to decide if being called the redskins instead of just the skins worth that.