From Nola.com:

” ‘Who Dat’ we do not claim to own by itself,” said Brian McCarthy, a spokesman for the NFL. “It’s when ‘Who Dat’ is used in conjunction with Saints marks that it’s a problem.”

McCarthy said T-shirts and items with ‘Who Dat’ and a fleur-de-lis logo unlike the one owned by the Saints are allowed as long as they are not advertised as being Saints or NFL paraphernalia.

‘Who Dat’ shirts being sold at the Fleurty Girl shop on Oak Street would be acceptable, McCarthy said, as long as the shop removes advertising referring to the Saints.