There are two ways to use waste of
cum.
1- In sub/dom relationships, you can call your sub partner a cum dumpster (a way to denigrate them). This frase is even worse. It can be interpreted that they aren't even worth being a cum dumpster. They are a "waste of
cum". You can even add a "waste of my
cum".
2- A way to refer to someone that is so worthless and useless that it's a waste that his/her/their father wasted their
cum in making him/her/them.
"Come on love!
Cum on my chest" "No, you bitch. You're a waste of (my)
cum!
"You're such a fucking failure. You're a fucking waste of
cum."