This is one of the expressions used to refer to microcars, the ones that don't require a driving license. This term is usually used in Alentejo (a province in the center-south of Portugal). We call it "mata-velhos" because, usually, elderly people are the ones who drive microcars and because of this, they're more prone to having accidents driving microcars (or being hit by them).
In northern regions we call it "papa-reformas" (pension-eater) because they're very expensive and one can spend their entire pension money paying for it.
"Olha, vai ali um mata-velhos!"
"Look, there goes an old-people-killer!"