The meal, especially dinner, is served. Refers to any food being served, not just soup. OK, gang, soup's on! Please wash your hands and come to the table! A: "Soup's on, Bill!" B: "OK, honey. I'll be there in two minutes."
See also: on
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
in. Dinner is ready. Time to eat! Soup’s on!
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.