Maybe you meant centuries, and not millennia? English, even it's oldest forms, hasn't been around for millennia. Anyway, as your own etymology link demonstrat

Maybe you meant centuries, and not millennia? English, even it's oldest forms, hasn't been around for millennia. Anyway, as your own etymology link demonstrates, the word's first use in its present meaning, as an executive leader of a republic--especially of something the size and importance of a nation state--is first in the US Constitution. Prior to that it generally meant leaders of more modest organizations: "heads of religious houses, hospitals, colleges and universities."