The NFL, that's who.

The league sent letters to Fleurty Girl and Storyville, two retail shops in New Orleans, telling them to cease-and-desist selling merchandise with the "Who Dat" phrase, claiming it violates federal trademarks held by the Saints franchise. According to the NFL, the Saints registered the phrase as a trademark in 1988 with the Louisiana secretary of state.

In case you didn't know, the Saints and their fans chant "Who dat say dey gonna beat dem Saints!" after wins. To hear it in action, click here.

Fluerty GIrl was selling t-shirts that had "#WHODAT" printed on the front. The "#" sign is used on Twitter to group tweets by a specific topic.

Said store owner Lauren Thom: "I was surprised. I think everybody was. It was designed to unify the Who Dat Nation, not within a tweet, but through a shirt."

Thom began selling the shirts in August on her website and then opened her store two months ago.

Storyville is apparently in violation of the trademark because its black t-shirts use the phrase "Who Dat Nation," and "Who Dat" along with XLIV, the roman numeral for this year's Super Bowl (another NFL trademark).

According to the NFL's letter to the stores: "...any combination of design elements (even if not the subject of a federal or state trademark registration), such as team colors, roman numerals and other references to the Saints," are also trademark violations.

A very broad interpretation at best.

Interestingly, the phrase was introduced to football fans by former Met Ron Swoboda, who landed in New Orleans as a sportscaster in the early 1980s, though he had no previous connection with the area.

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The Saints registered the phrase as a trademark in April 1988, and in May, trademarked it when used with the team's fleur-de-lis logo as well. But natives of New Orleans say the phrase dates back to a cheer used by a local high school decades ago, and may have roots all the way back to minstrel shows in the late 1800s.

Two candidates for a Senate seat -- incumbent David Vitter (Rep.-La.) and his Democratic challenger, Representative Charlie Melancon (D-Napoleonville) -- are both fighting the league on this topic. Vitter went so far as to pen a letter to the league.

Included in it was this paragraph: "This letter will also serve as formal legal notice that I am having t-shirts printed that say 'WHO DAT say we can't print Who Dat!' for widespread sale in commerce. Please either drop your present ridiculous position or sue me."

Game on NFL.