“Serious as a snake bite” (“Serious as a snakebite”)

“Serious as a snake bite” (or “serious as a snakebite”) means very serious, indeed. A snake bite could cause serious illness or death—and Texas has plenty of snakes.
 
The phrase “serious as a snakebite” has been cited as a Texas expression by at least 1996.
 
 
25 February 1996, Dallas (TX) Morning News, “Piney Woods history buffs talk Texan” by Kent Biffle:
Some Texas expressions are plain foolishness. Others are serious as a snakebite.
 
13 December 1996, Fort Worth (TX) Star-Telegram, “Cowboy Kudos,” pg. 8:
Congratulations, Cowboy. When we say you deserved to lasso the award, we’re serious as a snake bite.
     
Google Books
One tough Texan
By M. J. Rodgers
Toronto: Harlequin Books
1997
Pg. 148:
“You’re not serious.”
“Just as serious as a snake bite.”
   
Google Groups: alt.support.tall
Newsgroups: alt.support.tall
From: Margaret

Date: 1999/09/30
Subject: Re: Incredible shrinking bathrobe
 
Linda, I was ‘serious as a snakebite’, as they say in West Texas!