“Grinning like a jackass eating briars” means someone who gives a very wide smile and shows a lot of teeth. “Grins at her across the caldron after the fashion of a horse eating briars” was cited in print in 1887. “Grinning like a jackass eating briars” was cited in print in 1936.
 
The saying is popular in the American South.
   
 
Chronicling America
16 July 1887, The Ouchita Telegraph (Monroe, LA), pg. 4, col. 2:
Keren is brewing some sack for the fond father, when up comes Mister Peter Mouldy with his knitting and grins at her across the caldron after the fashion of a horse eating briars.
   
Google Books
Butcher Bird
By Reuben Davis
Boston, MA: Little, Brown
1936
Pg. 136:
She could see Slim (Pg. 137—ed.) yet standing laughing like a jackass eating briars while her cat shot off across the fields and never did come back.
Pg. 164:
Dora eyed him suspiciously, and he stopped at the steps grinning like a jackass eating briars, working his lips and flashing his teeth.
 
23 January 1941, Galveston (TX) Daily News, “Here’s the Dope” by Albert Reese, pg. 8, col. 1:
Uncle Billy suddenly grinned like a jackass eating briars and made a jump for us.
 
21 October 1944, Greensboro (NC) Daily News, “Among Us Tar Heels” by W. T. Bost, pg. 4, col. 5:
The farmer said he would not support a man who looked like a mule “which has been eating briars.”
   
Google News Archive
2 March 1950, The Robesonian (Lumberton, NC), “Raleigh Roundup” by Eula Nixon Greenwood, pg. 8, col. 1:
In the early days of little February many a Frank in North Carolina was grinning like a horse eating briars.
 
2 April 1955, Omaha (NE) World-Herald, “The Oafs Created by Our Speech” (Lydel Sims in Coronet), pg. 12, col. 3:
He was just standing there, grinning like a jackass eating briars.
 
Urban Dictionary
mule eating briars
Grinning from ear to ear, displaying all your teeth, much like a mule eating briars would look. Not to be confused with a shit eating grin.
Shirley came out of her alimony hearing lokkin like a mule eating briars.
by jamesnbk1108 Feb 8, 2009
 
Twitter
Sheila Morrison‏
@SheilaRM  
visit http://www.southernsoup.com and find out something about the south .....southern saying…..she is smiling like a “mule eating briars”
12:11 AM - 9 Oct 11
 
Twitter
Kristi KK Schille
r‏@kkschiller  
There is an old Texas saying “grinning like a jackass eating briars.” I give you, @JoeBiden #VPDebates
7:44 PM - 11 Oct 12 from Houston, TX
 
Google Books
Mystery in the Little Woods:
Spinning the Yarn, Book Two
By Pam Donham Suitt
Bloomington, IN: Booktango
2013
Pg. ?:
She was grinning, as my grandma would say, “like a mule eating briars.”