If something is manifestly true, wrong, unfair, or better, it is plain: you can see it or understand it without a long argument. “He is manifestly tired” means anyone looking can tell. It is a careful, slightly old-fashioned word. Teachers, judges, and newspaper writers use it more than friends on a phone. Though actually, people sometimes drop it in just to sound sure, even when the thing is not quite that obvious — so the tidy meaning is “clearly, out in the open,” but real use can be a bit of showing off. Do not mix it with the new, trendy use of manifest (“I will manifest a new job”), which is a different, looser idea about wishing something into being. Manifestly is not that. It is the old family: something already there, easy to see.