lose one's lunch - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Vomit, as in When Anne saw the wound, she thought she'd lose her lunch. This expression does not usually refer to a specific meal and probably survives because of alliteration. [Colloquial; c. 1940]