One confused search through a dictionary of idioms later . . .
Leif: I’m so sorry! I should have guessed you meant a literal arrow.
Thorn: Don’t say sorry! I learned something new.
Smartcrystal: “take an arrow to the knee” (slang): to become engaged (to be married).
Thorn: It’s sad . . . Marriage, in Sønheim, is something disabling? In Ceannis, if it’s like that, you’re doing it wrong.
Leif: N-no! That isn’t where the expression comes from! It isn’t that literal! . . . well, it isn’t literal at all. But it isn’t that kind of figurative! . . . I think that’s the word I mean.
Thorn: Well, if you’re not sure . . .