The somewhat less unpleasant of two poor choices. For example, I'd rather stay home and miss the picnic altogether than run into those nasty people—it's the lesser of two evils. This expression was already a proverb in ancient Greek and appeared in English by the late 1300s. Chaucer used it in Troilus and Cressida.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.