“Pain is (just/only) weakness leaving the body” is a popular exercise adage seen on items such as posters and T-shirts at many gyms. The saying has been popular in the U.S. Marine Corps and has been cited in print since at least 1991. The Marine Corps has used the saying in advertisements and had it trademarked in 1998.
 
“Pain is weakness leaving the body” is similar to “no pain, no gain” and “what doesn’t kill you makes you stronger,” meaning that hardship strengthens one’s body and character.
   
   
Wiktionary: what doesn’t kill you makes you stronger
Proverb
what doesn’t kill you makes you stronger
1.Used to express the sentiment that hardship or difficult experiences build moral character.
 
Urban Dictionary
Pain is weakness leaving the body
“Pain is weakness leaving the body”
The above is a piece of propaganda used by the US Marine Recruiting office to get more people to join the marines. Its also figuratively true, and very effective in their commercials.
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by JokingClown Jan 14, 2007
 
6 June 1991, Washington (DC) Post, “In Quantico, You Can’t Get There From Here; Effort to Redo Addresses Hits Resistance” by Avis Thomas-Lester and Kathleen Kennedy Manzo, Metro, pg. B1:
Controversy doesn’t come often to the tiny backwater perched on the Potomac River and surrounded on three sides by Quantico Marine Base, 45 miles south of Washington. Except for the occasional brawl in the town’s eight restaurants, which serve a mainly military crowd, things are pretty quiet. The town’s major industries are barbershops specializing in crew cuts and variety stores selling T-shirts with slogans such as “Pain is Weakness Leaving the Body.” The proverb is an unofficial slogan of the U.S. Marine Corps; it is also common among “extreme” athletes.
 
Google Books
Dragon Sim-13
By Bob Mayer
Novato, CA: Lyford Books
1992
Pg. 32:
In the Rangers, Olinski had learned to ignore the pain and discomfort that hard, realistic training entailed. This hard-core quality endeared the man to Riley, who was fond of saying that pain was weakness leaving the body.
     
14 November 1995, Oshkosh Advance Titan (The University of Wisconsin), “Stars” by Huggy Bear, pg. 10, col. 1
This week you will be struck in the groin with a blunt object. Huggy says just walk it off. Pain is only weakness leaving the body.
 
8 June 1997, Pacific Stars and Stripes, “MPs tough it out at ‘Warfighters’” by Rich Roesler, pg. 4, col. 2:
“Pain is weakness leaving the body,” Smith declared. “That’s all it is.”
(Spc. Jerry Smith. The story is from Waegwan, South Korea—ed.)
 
10 December 2003, The Valley Independent (Monessen, PA), “Boot Camp comes to California” by Keren Hall and Kate Bennett, pg. 8A, col. 1:
Muscular marines in black shorts and T-shirts with the slogan “Pain is weakness leaving the body” led students and faculty members through a rigorous fitness routine.
     
New York (NY) Times
August 20, 2008
A Sunday at the state fair
By Garrison Keillor
I got to go to the Iowa State Fair on Sunday and eat a very excellent pork chop on a stick as I stood by the U.S. Marines booth, where various civilians lined up to do chin-ups on a high bar, counted off by a Marine whose T-shirt said “Pain Is Weakness Leaving The Body.”
 
(Trademark)
Word Mark PAIN IS WEAKNESS LEAVING THE BODY
Goods and Services IC 035. US 100 101 102. G & S: Employment recruiting consultation; Providing employment information; Providing on-line employment information in the field of the military; Recruitment and placement of personnel in the field of the military. FIRST USE: 19981231. FIRST USE IN COMMERCE: 19981231
Standard Characters Claimed
Mark Drawing Code (4) STANDARD CHARACTER MARK
Serial Number 85640986
Filing Date June 1, 2012
Current Basis 1A
Original Filing Basis 1A
Owner (APPLICANT) U.S. Marine Corps, a component of the U.S. Department of the Navy n/a agency of the united states government UNITED STATES Room 4B548 3000 Marine Corps Pentagon Washington D.C. 20350
Attorney of Record Philip J. Greene
Type of Mark SERVICE MARK
Register PRINCIPAL
Live/Dead Indicator LIVE