An unpleasant, mean person, as in We cautioned Bill about his friend, who was clearly a bad sort. The antonym is a good sort, a pleasant, kind person, as in She's a good sort, always helping her neighbors. The latter is stronger than not a bad sort, as in He seems ill-tempered now and then, but he's not a bad sort. All three terms use a sort in the sense of “kind of person.” [ Second half of 1800s ]