[Robert A Cook PE]

It’s not a “dress”. It’s a t-shirt. And with probably nothing else under it.

The high school dress code can’t be enforced (as seldom as they are even enforced at all any more) for teenagers attending if “parents” are allowed to show up in worse.

[Tired of Taxes]

It's called a "t-shirt dress":

Other examples of t-shirt dresses

And more examples

It's not something I would wear... but I'm older.