When my mother sang this song to me (I was 3 or 4, let's not go into details), she told me that "La C" was a jeep that had run out of gasoline. (Cassells Spanish Dictionary translates gasoline as "gasoleno" or "gasolina") My mother used the femenine form, and kept "fumar" (to smoke) as the verb, which can make sense if you stretch it a bit.