bottom drops out, the - 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.
Also, the bottom falls out. A collapse occurs, as in The bottom dropped out of the steel market, or When they lost the game, the bottom fell out of their hopes to make the playoffs. This term alludes to collapsing deeper than the very lowest point, or bottom. [First half of 1600s]