To look or to feel like “death eating a cracker” is not to look or to feel well at all. Death always looks like death, despite eating a cracker, a cookie, a sandwich, or anything else. The expression “like death eating a cracker” means the same as “like death on toast” and “like death warmed over.”
 
“Like death eating a cracker” has been cited in print since at least 1949. Ralph Ellison’s novel Invisible Man (1952) included the food variant “looking like death eating a sandwich.” The author Rita Mae Brown has used “like death eating a cracker” in several of her novels. “Like death eating a cracker” has been reported to be a Southern expression, especially in Kentucky.
 
   
Wiktionary: like death warmed over
Adjective
like death warmed over (comparative more like death warmed over, superlative most like death warmed over)
1. (simile) Ill, unwell.
Usage notes
. This phrase is usually used with verbs like feel or look.
. The equivalent form like death warmed up exists well.
       
Urban Dictionary
death eating a cracker
A phrase to describe somebody who looks so vile, dried up and old that they are near death. Because the only thing that is worse then looking like death is looking like death attempting to eat a food that dries you up.
Dude your neighbor looks like death eating a cracker.
by 7fingersphil May 5, 2008
     
Google Books
A.A. Grapevine
Alcoholics Anonymous
Volume 6
1949
Pg. 31:
... meditation is a fine thing but he can’t see for the life of him why, when a fellow’s meditating, he has to look like death eating crackers. Don K., San Francisco, California.
   
Google Books
Invisible Man
By Ralph Ellison
New York, NY: Random House
1952
Pg. 562:
And man, that crazy sonofabitch up there on that hoss looking like death eating a sandwich, he reaches down and comes up with a forty-five and starts blazing up at that window— And man, talk about cutting out!
 
Google Books
The voices of Glory
By Davis Grubb
New York, NY: Scribner
1962
Pg. 185:
It was a face like Death eating a cracker and the smallpox scars I’d had since I was twelve looked deeper than ever.
   
Google Books
The New Writing in the U.S.A.
Edited by Donald Allen and Robert Creeley
Harmondsworth: Penguin Books
1967
Pg. 112:
You look like death eating a soda cracker.
(The LeRoi Jones play Dutchman—ed.)
       
Google Books
The Eavesdropper
By Peter Boynton
New York, NY: Harcourt, Brace & World
1969
Pg. 188:
Polson slept under a bush down the road and used Cy’s moving pads as blankets, and looked red-eyed and sharp-nosed, and rough, in the mornings. Like death eating a cracker.
   
Google Books
Goodnight ladies: a novel
By Babs H. Deal
Garden City, NY: Doubleday
1978
Pg. 106:
“My God,” she said. “I look like death eating a cracker.”
   
Google Books
Six of One
By Rita Mae Brown
New York, NY: Bantam Books
1999
Pg. 318: 
No wonder Louise used to look like death eating a cracker on bad days.
   
Google Books
Loose Lips
By Rita Mae Brown
New York, NY: Bantam Books
1999
Pg. 72:
I mean, I looked like death eating a cracker and you looked, well, not yourself.”
   
A Way with Words - Discussion Forum
wordelf
1:10AM
May-07-10
Like death eating a cracker:
 
Two other phrases come to mind, similar in that they involve food and death, but different in that in these expressions, death is not the eater but the eaten:
 
1. “You look like death warmed over.”
2. “How do you feel?” “Like death on toast.”
 
I agree with Elysia that these phrases connote queasiness, or that strange, hollow feeling your stomach may get when you have been deprived of sleep. #2 was a pretty common expression at Valparaiso U. in Indiana when I was there (6 years ago) between people commiserating about staying up way too late writing papers. “Death warmed over” just sounds nauseous– at least, it makes me think of someone using the iffy leftovers that have been languishing in the fridge; by analogy, the person you say this to looks like he or she is but tenuously alive, or like a walking stiff.