“Here I sit, cheeks a-flexin’, giving birth to another Texan” is bathroom graffiti seen in New Mexico and Oklahoma. They really must be entertainment-deprived in those states.
 
The potty poetry dates from at least 1984, and the author is unknown (of course).
 
 
Urban Dictionary
texan
Synonym for feces or stool.
Here I sit cheeks a flexin’ just gave birth to another Texan.
tags shit crap aggie longhorn know it all
by Black Duanie Cuntlap, CA Oct 4, 2006
   
Bathroom Scrawlings
“Here I sit buttocks a flexin’
givin birth to another Texan.”
submitted via email; location not specified
 
Restroom Graffiti
Here I sit, buns a flexin’
Givin’ birth to another Texan
—Lolly in Philmont, New Mexico

Daily Kos: Is Texas Necessary?
And the most common toilet graffiti in New Mexico:
Here I sit, cheeks a flexin’
Giving birth to another Texan
 
I’m sure there’s more . . .
by Deward Hastings on Fri Sep 01, 2006 at 06:47:17 PM PST
 
Google Groups: net.jokes
From:  Walt Pesch
Date:  Wed, Jan 25 1984 4:56 pm
 
There’s the one from the bathroom wall:

Here I sit, my cheeks a flexin’,
I just gave birth to another Texan.
 
Google Groups: net.jokes
From:  Dan C. Duval
Date:  Tues, Feb 7 1984 6:04 pm
 
Once encountered on an outhouse wall in Oklahoma:
 
Here I sit,
Cheeks a flexin’,
Giving birth
To another Texan.
 
Those Oklahomans really don’t like Texans.