In “Parable of the Prodigal Son,” what does prodigal mean and which brother is it?
The word "prodigal" means someone who wastes what they have, someone who is very wasteful. From that, you can see which son it was that was prodigal. It was the younger son who is referred to in the "title" of this parable.
If you remember, it was the younger son who wanted his inheritance right away. He went off and he spent the inheritance in really stupid ways -- he totally wasted it. Meanwhile, his older brother stayed at home doing everything just the way he was supposed to. So he was definitely not prodigal.