When I was reading harry potter and Percy Jackson crossovers, I realised something.

Snape is a son of Zeus.

It makes sense tho!

Snape is the half blood prince, yeah? Half blood is easy to guess. He's a demigod.

Prince? Zeus us the king if the gods, ja?

Sons of kings are called princes. Snape is the son of Zeus.

Hence the term, half - blood prince.

Mindblown.