It says "degrees of / freedom!!".
Why it is funny: because there is a famous meme on the web about the Braveheart movie (starring Mel Gibson as the Scottish independence hero William Wallace), especially the scene with all the yelling "freedom!" before the battle. Degrees of freedom are inversely linked to the number of parameters of a model (its parsimony if few). An illuminating example would be to claim that what goes under Wallace's kilt has a lot of degrees of freedom.
My personal favourite.
This T-shirt is made with 100% coton, ringspun technology. It has an exceptionnaly high thickness, making it both robust and pleasant to wear. The design is manually head-transfered to the fabric via the flex technology. This T-shirt can be washed inside-out at 40°C. No tumble dryer. If you damage the printed layer by accident (or tumble dryer), iron it through a parchment paper to smooth it again.