I’ve never looked it up before but in my head it has always meant “which is also a” or “which has turned into a.” I guess I was off a little but it is a very rarely seen expression. Probably because of the cum-cum-jizz association.

It's the Latin preposition cum ("with"), used in hyphenated compounds to describe something that's two things at once. Examples:

• Kitchen-cum-dining room: a room that serves as both
• Bed-cum-sofa: a sofa that converts to a bed
• Secretary-cum-bodyguard: one person filling both roles