The “T” Refers to the Shape

The answer is actually very simple. The “T” in “T-shirt” refers to the shape of the shirt.

When a T-shirt is laid flat, the body of the shirt forms a straight vertical line, while the sleeves stretch out to both sides. Together, they create the shape of a capital letter “T.” That is where the name comes from.

It is not short for a secret word. It does not stand for “training,” “textile,” or “top.” It simply describes the appearance of the shirt when viewed flat.