It's not like I'm a fan of the concept itself, but I feel like "at will" is not really a euphemism, but just a straightforward statement of the conditions under

It's not like I'm a fan of the concept itself, but I feel like "at will" is not really a euphemism, but just a straightforward statement of the conditions under which you can quit or be fired: at will, in other words, because your employer or you say so.