“U Signed the Motherfucking Contract” is a backronym (back acronym) of “USMC” (United States Marine Corps). “U Signed the Motherfucking Contract” has been cited in print since at least May 1995.
 
Other backronyms of “USMC” include “Uncle Sam’s Misguided Children,” “Uncle Sam’s Motorcycle Club,” “Uncle Sam’s Mountain Climbers.” “University of Science, Music and Culture,” “Unlimited Shit and Mass Confusion” and “U Suckers Missed Christmas.” Popular “Marine” backronyms include “Muscles Are Required, Intelligence Not Essential” and “My Ass Rides In Navy Equipment.”
 
 
Google Groups: sci.military.naval
Derek Neil Condra   5/17/95
Marine Soliders. How about Marine Jokes?
(...)
USMC:         
U Signed the Motherf***ing Contract
This is good when a Marine complains about something,
as in:
Marine Pvt: “Damn, I don’t want to pull firewatch again.”
NCO:      “Hey, remember, U.S.M.C”
 
Google Groups: rec.humor
A Long Friday From The Field
Dark Shadow  
5/30/98
(...)
USMC
U Signed the Motherfuckin’ Contract
   
Citypaper (Philadelphia, PA)
November 11-17, 2004
Battle Scars
A fledgling anti-war outfit sets up shop in Philly.
by Mike Newall
(...)
He’d been against the war from the start. It was an unjustifiable, unneeded war fought for oil rather than freedom, but as they say in the United States Marine Corps: U Signed the Motherfuckin’ Contract.
 
Google Books
This Recruit:
A Firsthand Account of Marine Corps Boot Camp, Written While Knee Deep in the Mayhem of Parris Island
By Kieran Michael Lalor
Bloomington, IN: iUniverse
2010
Pg. 90:
As Drill Instructor Sergeant Willis always says quite poignantly, “USMC stands for U Signed the Motherfucking Contract.”