Language deemed inappropriate, derogatory, or profane will not be accepted
In other words, if you are attempting to order a New Orleans Saints #00 replica jersey with the name Fuckmook on the back, you are basically shit out of luck.
Also of note from the NFL Shop site:
Linking to this Web Site. Creating or maintaining any link from another Web site to any page on this Web Site without our prior written permission is prohibited. Running or displaying this Web Site or any information or material displayed on this Web Site in frames or through similar means on another Web site without our prior written permission is prohibited. Any permitted links to this Web Site must comply will all applicable laws, rule and regulations.
Update: Leigh has inspired me to seek graphical representation of the concept.