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A seemingly small but actually crucial weakness.
[From Achilles' being vulnerable only in the heel.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A weakness that may not seem important but is actually mortal. The ancient Greek hero Achilles was made invulnerable by being dipped in the river Styx as a child. His mother held him by the heel, which was not dipped, and it was a wound there that killed him.
Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited