“Bring it on!” is so common that no definitive origin can be traced. On the American Dialect Society list, word researcher Grant Barrett wrote: “I’ve checked the usual places, but the phrase is really too common to come to any real head, and difficult to search for in the databases.” Douglas G. Wilson thought that the phrase might have come from “bring him on,” a phrase meaning that a boxing challenge is accepted.
 
The movie Bring It On (2000)—about high school cheerleading—helped to popularized the phrase. In July 2003, President George W. Bush issued a challenge to America’s enemies: “Bring ‘em on!”
 
The phrase is often shortened to just “Bring it!” and is a popular declaration of an accepted challenge. Despite President Bush’s usage and the Texas “wild west” flavor to the language, there is no basis to assume that the common phrase comes from Texas or anywhere else.
           
     
Urban Dictionary
bring it
Used as a “manly” come back to someone who is either being challenged or it can be offered as a direct challenge to another;
Similar expressions:
“Show me what you got!”
“Do your worst”
“Bring your shit!”
“Go for it!”
Basketball Player 1: “Im gonna slam dunk yo punk ass.”
Basketball Player 2: “Ahhhight, bring it bitch. Let’s see what you got!”
by toadcheese Feb 25, 2004
 
Wikipedia: Bring It On (film)
Bring It On (2000) is a film about a high school cheerleading squad, starring Kirsten Dunst, Eliza Dushku and Gabrielle Union. It was directed by Peyton Reed and written by Jessica Bendinger.
 
A straight to video sequel Bring It On Again was released in 2003, sharing only producers with the original. A third film entitled Bring It On: All or Nothing was another direct-to-video sequel released in late 2006. A fourth film, tentatively titled Bring It On: In It to Win It, is currently in production.
 
USA Today
Posted 7/2/2003 11:47 AM   Updated 7/2/2003 9:07 PM
Bush: ‘Bring on’ attackers of U.S. troops
WASHINGTON (AP) — President Bush said Wednesday that American troops under fire in Iraq aren’t about to pull out, and he challenged those tempted to attack U.S. forces, “Bring them on.”
   
3 July 2003, New York Post, “Bush: We Won’t Be Run Out of Iraq,” pg. 8:
WASHINGTON - A tough-talking President Bush yesterday said ambushes and attacks on U.S. troops won’t scare America out of Iraq and defiantly challenged foes: “Bring ‘em on!”
 
Bush said the United States has a “plenty tough force there” to deal with terrorists and henchmen of the toppled Saddam Hussein regime.
 
“There are some who feel like that if they attack us that we may decide to leave prematurely. They don’t understand what they’re talking about, if that’s the case,” Bush told reporters at a photo op.
 
“There are some who feel like - that the conditions are such that they can attack us there. My answer is, bring ‘em on!”
 
He insisted: “We’ve got a plenty tough force there right now to make sure the situation is secure” - but added that “we always welcome help” from other countries. 
 
5 July 2003, The Virginian Pilot, “Saloon talk at the White House” (editorial), pg. B8:
At a White House press event on Wednesday, President Bush goaded militants who have been targeting U.S. troops in Iraq. Commenting on the attacks, Bush said, “Bring ‘em on. We’ve got the force necessary to deal with the security situation.’’
 
Coming from a president who campaigned hard on the need to restore dignity and respect to the White House, this is both odd and disappointing.