Capsize, turn upside down, as in When they collided, the car turned turtle. This expression alludes to the helplessness of a turtle turned on its back, where its shell can no longer protect it. [First half of 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.