contradiction in terms
A statement that seems to contradict itself, with one part of it denying another. For example, I've always believed that “a poor millionaire” was a contradiction in terms. [Late 1700s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.