Should be used either when a person keeps repeating the same complaints over and over again and seems unhappy in the relationship and unwilling to do anything about it, or when anyone can see at a glance that it's a toxic, if not abusive, relationship.
Lindsey: "Aargh, I'm so frustrated with Jon, he never calls when he says he will and keeps telling me about what other girls are hot! What should I do???"
Rachel: "Well, considering you've been doing nothing but tell me about how much you hate Jon for the past three months, you should either tell him this, or DTMFA. We're all sick of hearing it!"
Alex: "Yeah, I'd love to come hang out with you guys, but Jamie will totally lose it if I talk to another girl. Last time I went somewhere without her she locked me out of the house and wouldn't talk to me for two days."
Leighanne: What the hell? She sounds like a total psycho. You should DTMFA before she boils your bunny or something.