1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): Hang on to your hat! [and] Hold on to your hat!
Fig. Get ready for what’s coming!; Here comes a big shock! • There is a rough road ahead. Hang on to your hat! • Here we go! Hold on to your hat!
McGrawhill's American Idioms And Phrasal Verbs