According to the Harbaugh family, father Jack Harbaugh was an assistant football coach at the University of Iowa in 1972 when he told his sons Jim (born in 1963) and John (born in 1962) this advice:
 
“Attack this day with enthusiasm unknown to mankind, and don’t take any wooden nickels.”
 
Jim Harbaugh (head coach of the San Francisco 49ers) and John Harbaugh (head coach of the Baltimore Ravens) would face each other in the 2013 Super Bowl. Jim Harbaugh often repeated the saying to the press, using it while coaching Stanford University (2007-10) and then the 49ers (in 2011).
 
“You must go at your tasks each day with enthusiasm” was cited in print in 1917; “Approach each day with enthusiasm and the desire to make it more interesting and fruitful” was cited in print in 1937. Jack Harbaugh’s “unknown to mankind” appears to be an original ending to the common saying.
 
 
Wikipedia: Jack Harbaugh
Jack Avon Harbaugh (born June 28, 1939) is a former American football player and coach, and the father of the first pair of brothers to serve as NFL coaches and the first pair of head coaching brothers to face off in a Super Bowl: Jim and John Harbaugh.
 
Wikipedia: Jim Harbaugh
James Joseph “Jim” Harbaugh (/ˈhɑrbɔː/; born December 23, 1963) is an American football coach and former quarterback who is the head coach for the San Francisco 49ers of the National Football League (NFL). He played his college football at the University of Michigan, and was an NFL quarterback from 1987 to 2000. He has been a football coach since 2004, and first served as a head coach in 2004 for the University of San Diego. After coaching Stanford football from 2007 to 2010, he became San Francisco 49ers head coach in 2011.
 
Wikipedia: John Harbaugh
John Harbaugh (born September 23, 1962) is the head coach of the Baltimore Ravens. Previously, he coached the defensive backs for the Philadelphia Eagles and served as the Eagles special teams coach for nine years. Harbaugh and his younger brother, San Francisco 49ers coach Jim Harbaugh, are the first pair of brothers in NFL history to serve as head coaches. John and the Ravens will face off against Jim and the 49ers at Super Bowl XLVII in New Orleans on February 3, 2013.
 
8 August 1917, Rockford (IL) Republic, “Health Beats Wealth,” pg. 9, col. 7:
In order to make a success, you must go at your tasks each day with enthusiasm.
(...)
—Bernarr Macfadden in August Physical Culture.
   
Google News Archive
10 March 1937, Painesville (OH) Telegraph, “Once Overs” by J. J. Mundy, pg. 6, col. 6:
Approach each day with enthusiasm and the desire to make it more interesting and fruitful.
   
Palo Alto (CA) Online
Uploaded: Tuesday, December 19, 2006, 9:50 PM                              
Paly grad brings home ‘an enthusiasm unknown to mankind’
‘The journey ... from Palo Alto High School ... to The Farm has prepared me well.’
An excited Jim Harbaugh promised Tuesday afternoon to attack his new job as Stanford University’s head football coach “with an enthusiasm unknown to mankind.”
 
A Day in the Life of PF3
Wednesday, May 9, 2007
Attack Each Day with Enthusiasm
Yesterday is but a memory.
         
Tampa Bay (FL) Times 
Tampa Bay Buccaneers have tough road trips ahead
By Rick Stroud, Times Staff Writer
In Print: Sunday, October 9, 2011
(...)
SAN FRANCISCO TREAT: 49ers coach Jim Harbaugh finally can have contact with Bucs backup QB Josh Johnson today, at least with a handshake.
(...)
“Just in life, in general, he’s always said, ‘Attack each day with enthusiasm unknown to mankind,’” Johnson said. “When you’re younger, you just think I’ll just go out and be high-energy, high-spirited.
 
“Now I look at it as going out and taking advantage of every opportunity and never letting a minute slip in regards to your preparation.”
 
The Bear Insider
82gradDLSdad
I certainly thought Harbaugh was a clown - 10-17-2011, 02:10 PM
When someone spews sayings like “We’re going to attack each day with an enthusiasm unknown to mankind” shortly after getting his first D1 coaching gig you have to think, “This guy is a clown.” Hell, he may still be a clown but he is a great football coach. Now I think his statement above isn’t so much clown-like but just a bit of hyperbole.
 
Sports Illustrated
March 26, 2012
Look Who’s Back
Coming off three straight 20-loss seasons for the first time in school history, Indiana returns to the Sweet 16
KELLI ANDERSON
The box of gray T-shirts arrived in Portland last Thursday, a few hours before Indiana tipped off against New Mexico State. The front of the tees read: ATTACK THIS DAY WITH AN ENTHUSIASM UNKNOWN TO MANKIND!—a mantra familiar to anyone who has spent time around Jim or John Harbaugh, the NFL coaches and brothers-in-law of Hoosiers coach Tom Crean. The shirts, which a handful of Indiana players wore the day after beating the Aggies 79—66, were a gift from John, who had added on the back: NCAA TOURNEY 2012 FINISH THE FIGHT.
 
New York (NY) Times
Teacher’s Grade Hinges on Student’s Playoff Test
By JOHN BRANCH
Published: January 11, 2013
(...)
In the locker room and on the practice field, Harbaugh is high energy. “Attack each day with an enthusiasm unknown to mankind” is an axiom he brandishes regularly, handed down from his father, Jack, a longtime football coach. (John Harbaugh, Jim’s brother, coaches the Baltimore Ravens.)
   
San Jose (CA) Mercury News
Harbaugh parents will be ‘neutral’ for ‘Lombardi Trophy game’
POSTED BY CAM INMAN ON JANUARY 24TH, 2013 AT 8:52 AM
(...)
Jack Harbaugh shed more light on one of the family’s now-famous mottos, and it dated back to his days as a University of Iowa assistant in 1972: “We’d take the two boys to elementary school. They were cold, hats pulled over their ears and didn’t look happy. I’d look at them in the back of the car and saw their sad faces. I said we’ll attack this day with enthusiasm unknown to mankind, and don’t take any wooden nickels.”