“Load your brain before you shoot (off) your mouth” means the same thing as “think before you act.” The “load your brain” saying has been cited in print since at least 1998 and is of unknown origin. A 2010 citation (below) claims that this is “an old Texas saying,” but there is no evidence that the saying is either old or from Texas.
 
     
Google Groups: alt.religion.wicca
Newsgroups: alt.religion.wicca
From: .(JavaScript must be enabled to view this email address) (StormFyst)
Date: 1998/05/31
Subject: Re: Satan is damed to the Lake of Fire!
 
Next time, load your brain before you shoot off your mouth.
   
Google Groups: alt.music.billy-joel
Newsgroups: rec.music.beatles, rec.music.artists.beach-boys, alt.music.billy-joel
From: “Bob Hedin”

Date: 1999/07/27
Subject: Re: Spam CD
 
Oh Carey, how you do prattle on… please load your brain before you shoot off your mouth.
       
Voice in the crowd
Friday, June 24, 2005
Nobody Thinks Anymore
(...)
There’s an old saying: “Make sure you load your brain before you shoot off your mouth.”
 
O.K. maybe it’s not an old saying. But it should be.
     
Google Books
Thoughts for Meaningful Life
By Pano George Karkanis
AuthorHouse
2008
Pg. 355:
Load your brain before you shoot your mouth.
 
Foreclosureblues
GARFIELD DEBATE FROM APRIL 2010
Posted on October 5, 2010
(...)
COMMENTS
Alina says:
April 10, 2010 at 3:30 pm
admin,
There is an old Texas saying – load your brain before you shoot off your mouth.