Being in a long-term relationship means that your partner gets to know you really well — which is a perfectly double-edged sword.

On the one hand, you can be yourself around them — your unfiltered, sometimes un-showered, lame-joke-telling self. On the other hand, they eventually learn better than anyone else exactly which buttons to push to set you off.

Hal Runkel puts it eloquently: "No one can touch you like the one you expose yourself most to, but no one can hurt you like the one you expose yourself the most to."

Runkel is a marriage and family therapist, and the author of multiple books on parenting and relationships, including, most recently, "Choose Your Own Adulthood."

When he visited the Business Insider office in May, Runkel shared his best advice for de-escalating a conflict that's spiraled out of control because one person said something that cut deep.

Actually, that advice is just one word: "Ouch."

It's a word that doesn't get used nearly enough in marriage, Runkel said.

Here's Runkel: "When [you're] in conflict, inevitably [you] will say something that hurts the other person using the 'inside information' that you have on them or that they have on you."

As in: Your partner knows you're struggling to lose weight and they blurt out, Of course you didn't take five minutes to walk the dog — you were too busy stuffing your face!

At that moment, Runkel said, "Everything in you wants to scream something right back at them: 'Oh yeah? Well, you're starting to look like your mother!'"

Here's where the word "ouch" comes in handy. Runkel explained that the best response in this situation is simply, "Ouch. That one hurt. I don't know if you were meaning to hurt me; I don't know if that's what you were going for; but that's what you did."

Your partner may get defensive and say something back like, "You've said some pretty hurtful things to me!"

Now here's your line: "You're right. I have, and I hate that I have."

"That conversation —which was a very familiar path, that fight — is now a totally different path because one of you chose to actually get vulnerable," Runkel said.

"It wasn't a step of pushing [your partner] away. It was a step of inviting [your partner] in by saying: You know what? I am open enough to you that you can actually hurt me. So now how about we talk to each other as if we actually love each other?"

That display of vulnerability is key — and a lot harder than it sounds.

Too many of us have this tendency to pretend that we're made of emotional steel — that when our partner insults us, or doesn't pay us enough attention, we're totally fine.

Ideally, when you implement Runkel's strategy, your partner will respond in kind, and you two can have a calm conversation about what's bothering you. Yet Runkel also mentioned that your partner's next sentence after you admit that you've been hurt will tell you a lot about your relationship.

All you can really do is start the dialogue: "That hurt. Tell me what you'd like me to do with that."