“Your stomach shouldn’t be a waist basket” (a pun on “waste basket”) is a saying that has appeared on many diet and nutrition websites. The term “waist basket” for “stomach” has appeared in print since at least 1931 and was a jocular “Daffynition” in 1948.
 
   
29 June 1931, New Castle (PA) News, pg. 4, col. 5:
Teacher: “Johnny, what is the most outstanding feature about your father?”
Johnny: “His stomach. It holds so much that we call it his waist-basket.”
 
1 February 1948, Seattle (WA) Daily Times, “Daffynitions” by Paul H. Gilbert, Magazine Section, pg. 13, col. 2:
STOMACH: A waist basket.
   
Google News Archive
11 July 1953, The Dispatch (Lexington, NC), “Try and Stop Me” by Bennett Cerf, pg. 7, col. 1:
Admitted that “women now travel at a pretty dizzy pace,” observes Colonel Duffy, “but fifty years ago they had more bustle.” He also defines “stomach” as a waist basket; a “hock shop,” a loansome place; and so on ...
 
22 September 1954, Augusta (GA) Chronicle, pg. 1, col. 1:
COMIC DICTIONARY
STOMACH
A waist basket.
(Copyright, 1954, by Evan Esar.)
 
Google NewsArchive
5 December 2008, Hildago County Herald (Lordsburg, NM), “Tiny tastes can total big calories duringthe holidays” by Tammy Dunlap, pg. 7, cols. 4-5:
Don’t let your stomach become a “waist” basket; use or freeze leftovers within 3 to 4 days.
   
Google News Archive
24 December 2008, Western Kansas World (Trego County, KS), pg. 18, col. 4:
Don’t Let Your Stomach Become a “Waist” Basket
by Tranda Watts
Extra calories sometimes sneak up on us over the winter holidays. They don’t always arrive in the form of large portions of calorie-laden food. Rather, they may tiptoe in through any tiny tastes throughout the day.
 
Google Books
Thinking Thin:
The Truth About Weight Loss
By Tom Nicoli
Kallisti Pub.
2007
Pg. 36:
Your stomach shouldn’t be a waist basket. —Anonymous
 
Google Books
All Diets Work, That’s the Problem!
By Jen Brewer
Rochester, MN: Petersen Press
2011
Pg. ?:
“Your stomach shouldn’t be the waist basket.” —Author Unknown