Perhaps from its being shaped like the letter T when spread out
American Heritage Dictionary of the English Language, 5th Edition
From its shape.
T-shirt Sentence Examples
Regardless of the type of coat you spring for, ensure that it is big enough to fit for the entire winter, and don't forget to try it on when your child is wearing multiple layers (a shirt and a sweater), instead of only a light T-shirt.
She wore a T-shirt and shorts that revealed her shapely, soft legs.
There she changed into jeans, a T-shirt and sneakers.
He wore a snug T-shirt that displayed the roped forearms covered in tattoos.
A tall, toned woman who looked like she did Pilates for a living stood in the hall in tight black leather pants and a tight pink T-shirt that drew attention to her large breasts.