"Arisa-chan, do you know にゃんにゃん?"
にゃんにゃん(literally "meow meow") is a Japanese slang meaning a sexual intercourse. I don't know how to translate this word (because I'm not a English speaker.)

Hmm, I had no idea. That certainly gives, umm, a new light on the whole catgirls thing.

Hm... if we want something cat related - and I think we do, since the followup comic post #228196 mentions cats again - the only thing that comes to mind is making a pun on "pussy". Or we could do a literal translation and explain the double entendre in a note.
But I'm not a native English speaker, either, so I might be missing a better option.

What does the little note in the margin at the bottom say? I can only make out 耳.

耳年増

年増 means a middle-aged woman (年=year,age/増=increase), and 耳年増 means "only her ears(=耳) are 年増", that is, a young woman who has no experience but much knowledge (generally about sex).

My opinion is to keep にゃんにゃん as 'meow-meow' since a number of the artist's work hinges upon characters not knowing what the phrase means and taking it literally.

Better keep it as "nyan-nyan", then.

I decide to use "nyan-nyan" as a Japanese word. Thank you all.

Here's an entry in a Japanese slang dictionary for "にゃんにゃん" for anyone interested: http://zokugo-dict.com/22ni/nyannyan.htm