"Do not disparage any person" means not even a non-Jew, as wrote the Tosphot, 'a' person means a Jew in particular and 'the' person a non-Jew as well. That is what "any person" (all persons) means, whether 'the' - a Jew, or 'a' – a non-Jew. The reason is brought by Tankhuma: "You are busy maligning your brother, defaming the son of your mother" - 'maligning your brother' Esau (Christians), leads to 'defaming the son of your mother' and one of your own nation. For if you accustom your tongue to speak disparagingly to the non-Jew you will speak thus of a wicked Jew, imagining that it is all the same, and then go on to the mediocre, and then on to the righteous – this leads to your mother's son. That is, this will lead to defaming your own mother's son. This is what is meant by 'you have no man who does not have his hour' – there will remain no one in this world without a time during which you will speak of them slanderously.