The tradition was codified in 2010, but New Orleans Saints fans have been crying out "Who Dat?" since long before then.
According to the Times-Picayune,the "Who Dat?" rallying call first originated during the fall of 1983, when WVUE-TV sports anchor Ken Berthelot and photographer Avis Landry were sent to capture video of a high school football game in St. Augustine. The city's Purple Knights had a pre-practice chant that went, "Who dat? Who dat? Who dat talk about beatin' St. Aug?" WVUE sports director Ron Swoboda loved the cheer and aired it on television on Sept. 1, 1983.
Three days later, the chant was heard echoing through the Superdome for the Saints' home opener against the St. Louis Cardinals.
The cheer became so popular among fans that the Saints officially adopted it during coach Bum Phillips' tenure. Aaron Neville, alongside local musicians Sal and Steve Monistere and Carlo Nuccio, recorded a version of "When the Saints Go Marching In" that incorporated the chant "Who dat say dey gonna beat dem Saints," performed by five Saints players.
In 2010, Drew Brees solidified its game-day usage with the introduction of a pre-game ritual. After the coin toss, a player or guest raises his or her hand over their head on the field. When their hand drops, fans in the Superdome break into three rounds of "Who dat! Who dat! Who dat say dey gonna beat dem Saints!"
Brees said in 2010 that he hoped the tradition would live on for decades after he's retired.
The chants have yet to cease in 2018, ringing loud through the Superdome as the Saints clinched the top seed in the NFC. Those cheers will be even louder on Sunday, when the Saints host the Rams for the NFC Championship.