What does it mean when someone says "fine" but isn't?

Usually that they've stopped arguing without stopping being upset. The word agrees; the flat voice, the sudden quiet and the quick exit don't.

It's often an early step toward what John Gottman calls stonewalling, when someone "completely withdraws from a conflict discussion and no longer responds to their partner." Gottman ties stonewalling to feeling flooded, so overwhelmed that shutting down is what's left. That makes it less a tactic than a limit, which is why pushing for more talk right then rarely helps.

Compare it with how they usually talk. Some people answer in single words all the time, and their "fine" means fine. What's worth noticing is a change during the conversation: someone who was arguing in full sentences a minute ago suddenly agrees to everything. A quick yes right after a charged exchange usually means the argument went underground, not that it ended.

Don't take it as agreement, and don't push for the real answer while they're overwhelmed. Say you don't think it's settled and offer to pick it up after a break. The Gottman Institute suggests the break last at least twenty minutes, which it says is how long the body takes to calm down. The steps are in How to take a break from an argument without stonewalling.

Say the true version instead, even a short one: you're too worked up to talk well right now and you want to come back to it. That turns a shutdown into a break, and it tells the other person the conversation isn't over.

Related: Stonewalling or just needing space · Red flags you're missing: 7 signs a conversation is about to go off the rails