When an animal, like a dog or a horse, gets very old, it may be constantly sick or in pain. The animal is "in misery." The owner may decide to end the animal's life, in a kind, comfortable, and humane way. A common way of explaining the decision is to say "I decided to put old Fido out of his misery." In this case, the speaker is using figurative language. It sounds like the speaker is breaking up a romantic relationship. Let's say Morgan says this to Dana. The literal meaning is "You can call me in nine months or maybe just kill yourself now." Morgan is being cruel. Morgan does not want to see Dana again, ever. Morgan says, in a nasty, teasing way, "you can call me in nine months," as if there were a chance of starting again. But Morgan then adds "or maybe just kill yourself right now," i.e. there is no chance at all.

put somebody out of their misery ​(informal) to stop somebody worrying by telling them something that they are anxious to know Put me out of my misery—did I pass or didn't I? Oxford learners dictionary Hope that helps