Most often they were given to men who had distinguished themselves as warriors, but other acts might earn one too, and sometimes women remarkably equal in First Nation cultures, but who were usually celebrated in other ways also earned a bonnet.
In short, the feathered headdress was a sacred vestment, much as the korowai (ceremonial feathered cloak) is to Maori.
Understandably, then, people of the Plains tribes are offended when they see others wearing imitation headdress that mocks the form of what, for them, is a special cultural treasure. Perhaps the distinction between whether this is racist or merely culturally insensitive depends on whether you know wearing one will give offence.
Dame Cooper it appears did not know, and has apologised for her ignorance. From a Western Eurocentric perspective this may still seem strange: if you make something in flattery of the genuine article, with good intent, surely that's a positive form of praise?
Not in the eyes of the Plains tribes. Given the genocide of the Indians by the early settlers, it is a very raw subject. Especially as in the US it is far too common for non-native people to make a display of wearing warbonnets, knowing they will give offence.
That's a racist act, and that's why fluffing for fashion is in Dame Cooper's case no excuse.
Moreover, it shows how deeply embedded racism is that such cultural misappropriation could be taken for granted. It is as much a statement of privilege as was Stephanie Key's near-nude selfie "art" piece a year or so back that portrayed her decorated in an elaborate pink mock-First Nations headdress and lace knickers - wrong on so many levels.
On that subject, I agree @Peace's "Kill the PM" song - if a semi-literate ramble over a repetitive bass lick can be called such - is trash and denigrates Stephanie as much as her father, but the right-wing bloggers resurrecting it some 18 months after it was released (to immediate oblivion) to try to prove the left is nasty too rather ices the dirty politics cake.
The point is that it's nowhere near sufficient to imagine all is right with the world simply because you ignore or refuse to acknowledge the wrongs.
As citizens we have a duty of care that demands we inform ourselves, for the betterment of all regardless of race or creed. Try hard to do that before you cast your ballot.
That's the right of it.
Bruce Bisset is a freelance writer and poet