hang on to your hat

Also, hold your hat. An expression warning someone of a big surprise. For example, Hang on to your hat, we're about to go public, or Hold your hat—we just won the lottery. This expression may allude, according to lexicographer Eric Partridge, to a wild ride on a rollercoaster. [Colloquial; first half of 1900s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.