This is an Original Grateful Dead vintage shirt from 1973. It's the iconic Jester shirt designed by Mouse Studios. The only flaw is tiny pinhole around the belly button and a couple tiny marks that should come out in the wash. But otherwise for being almost 50 years old it's still in great condition. Size is tagged medium but fit is more a small (pit to pit 17.5?
One thing to make note of - the sleeves are a slightly different, lighter shade of blue. I'm guessing it has something to do with how shirts were manufactured back then. Stitching inside suggests these are the original sleeves.