Half-witted describes a person, or something they do, as stupid — slow in the head, not thinking well. Though actually this is a describing word, not a name for a person: you say a half-witted plan or a half-witted remark. You do not say “he is a half-witted” on its own; the naming word for that insult is half-wit. It sounds old and sharp, a little like something snapped in a British film. People still understand it, but it is not a fair label, and it is not a medical word.