not a bad sort
see bad sort.
Not All Bad: 7 Ways “Bad” Can Be GoodPeople (and Thesaurus.com) have known for a while that sometimes being bad is really, really good—especially in the bad phrases below. Carry on.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.