warm as toast - 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.
Comfortably warm, as in It was freezing outside, but we were warm as toast in front of the fire. Despite the British custom of serving toasted bread in a rack that rapidly cools it, this idiom originated in England, at first as hot as toast (c. 1430) and by the mid-1800s in its present form.