A course of action with inevitable bad consequences has begun; there's trouble ahead. For example, Now the fat's in the fire—the boss arrived early and will see we haven't even started work. This expression, with its allusion to fat dropping into a fire and causing a burst of flames, was already a proverb in John Heywood's 1546 collection.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.