For Ezekiel and for editorials in newspapers, "whitewashing" something means trying to make a situation look better than it is—covering up the true nature of things. God throws this accusation at the false prophets who've been predicting that Jerusalem and Judah will survive the Babylonian assault:
I will break down the wall that you have smeared with whitewash, and bring it to the ground, so that its foundation will be laid bare; when it falls, you shall perish within it; and you shall know that I am the Lord. Thus I will spend my wrath upon the wall, and upon those who have smeared it with whitewash; and I will say to you, The wall is no more, nor those who smeared it—the prophets of Israel who prophesied concerning Jerusalem and saw visions of peace for it, when there was no peace, says the Lord God. (13:14-16)
The prophets have been whitewashing the truth about Jerusalem. Maybe they know what tends to happen to prophets who try to tell it like it really is. Nobody likes hearing about their horrible sins and imminent total destruction.