"This is… I recognize this," Rey said, looking at the hilt of the weapon. "This is a Sith dagger."
"A dagger used by the Sith?" Finn asked.
"No," Rey replied. "Not at all. Why would a Sith want a dagger?"
"...to… stab people?" Finn guessed. "Help me out here, Poe. That's something that happens with daggers, right?"
"I'm more someone who uses laser cannons," Poe replied. "But that sounds like something you do with a dagger… though I guess a Sith would have a lightsaber."
He glanced at Rey. "How about you explain?"
"It's a Sith dagger," Rey explained, picking it up. "It's an ancient Jedi tool."
"...no, you lost me again," Finn said. "Try again, this time more slowly."
Rey nodded, waving the dagger around. "It's a tool that's used by Jedi," she said. "So they can kill Sith."
"And it's… a Sith dagger," Poe said, warily. "Yeah, this is making sense."
"It's called a Sith dagger because it's for a Sith," Rey told them.
Two blank looks faced her.
"How about you remember that some of us haven't been reading ancient Jedi books," Finn suggested. "What's it for?"
"All right," Rey said. "So… part of being a Sith, specifically, is part of being a cult. It's the belief in the Sith way. And part of that idea is that to corrupt and make a Jedi fall to the Dark Side is a sweet revenge – in fact, it's something they would often try to actively seek out. They'd prefer to be victorious, but corrupting a powerful Jedi to ensure the cause lived on would often be worth their lives."
Poe made a face.
"Why does anyone ever join up?" he asked.
"Force influence, brainwashing, the uniforms?" Finn guessed. "I know it's pretty easy to think everything makes sense when you're in the middle of something."
"Right, right," Poe realized. "Sorry, bud… so that's the Sith, but, what about the dagger?"
"I'm getting there," Rey replied. "So a common thing the Sith would do is that, if disarmed or caught in a difficult situation, they'd make a point of how they were unarmed. Helpless. And that killing them would be killing an unarmed, helpless foe, and therefore a Dark side act. Something that would inevitably corrupt the doer."
She flipped the dagger into the air, catching it. "So. It's a Sith dagger. A dagger that a Jedi would throw to a Sith, making them no longer unarmed – and engraved with Sith runes, so that the Sith might be distracted by trying to translate the runes on the blade. You know. When the Jedi killed them."
"Sounds pretty grim to me," Finn muttered.
"Hey, light side doesn't have to mean stupid," Poe countered. "What do the runes say, anyway?"
"Don't try to rules-lawyer a force of nature, idiot," Rey answered. "Or, at least, that's what the books say the runes were on a traditional dagger."
She shrugged, pocketing it. "We might need this if I run into Palpatine."
AN:
From a friend's idea about what "Sith dagger" could mean.