A shirt that is unrealistically skin tight around the breasts, hugging the breasts on all sides. The shirt is impossible because normal clothes could never be this form-fitting without having two empty sacks where the breasts would go.
Not to be confused with taut shirt or tight shirt. A tight shirt is tight, but not skin tight around the breasts. A taut shirt is a shirt that has ripples from the fabric hanging taut between the breasts.