put on the feed bag
Also, tie on the feed bag. Eat a meal, as in Come on, it's time to put on the feed bag. This slangy term, alluding to a horse's feed bag that is literally tied on, dates from the early 1900s.
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.