Remus was horrible at sexual innuendos. That is he could never understand jokes that included such innuendos. And basically the main reason was that he didn't know the meaning of half that stuff.

(Or in other words, Remus Lupin was a cinnamon roll too precious for this world).

***

Remus: Hey, wait a moment. There's nothing funny in that, why are you guys laughing your heads off? It was a terrible joke.

...Silence in the dorm...

James: Don't tell me... Did you really not get that?

Remus: I have no idea what you all found so hilarious if that's what you mean by not getting it.

Sirius: Shhhh all of you, we have an urgent emergency... This kid needs to be schooled.

Remus: I what?

Sirius (smirking mischievously): Brace yourself, Remus John Lupin, for the strangest lecture of your life. And knowing you, it might even make you puke.

...Half an hour later...

Peter: What are you wondering, Rem?

Remus: I'm wondering where on Hogwarts could I get some holy water.

Sirius (in the background): Sorry to disappoint you, mate. No holy water here. But we could always get you some bleach.

Remus (scowling): Oh, I couldn't possibly thank you enough.

Sirius (laughing): You owe me for a lifetime. Even though it's unwillingly on your part.

Remus (annoyed): Sod off.