She meant "ONLY a T-shirt" as an above-waist garment. She observed that Oswald was not wearing another shirt or a jacket over the T-shirt. I have no sense of humor!!!
Psssst..I could crush a Grape ooooh yummy yummy.... I know what the poster intended to convey... But that's NOT what he wrote.....
When the person hearing a joke isn't smart enough to appreciate the joke and it needs to be explained to the dolt ...the joke loses it's humor....