Some digging later...
"Later, Tahani comes home to swap her gloves and Jianyu speaks.... (Another odd “English” phrase is used here, “Whisper in your snicker box .” A phrase never once heard over a 32 year period in that country. There are plenty of real quirky, to American ears anyway, British sayings that would have sufficed.)"(https://mikesfilmtalk.com/2016/09/30/good-place-jason-mendoza-forking-silence-golden-review/)
Keep two things in mind from this - the Good Place censors rude words, and allows for alliteration/rhyming etc. So one possible answer is that she was confused by the speaking and used a rude idiom. More likely we need to know what this part means.
"Snickerbox - urban dictionary
when a woman wipes her bottom from back to front. "
Ok so that almost likely. Yet it still doesn't make total sense. The conversation revolves around Jason speaking for the first time, so perhaps the event was mindbolling and she misspoke. Or she meant to say this and it had some reference to her parents who may have said something along those lines. Think back to one of your parents when your told off, they may have a unique phrase not said by many.
It could simply be a portmanteau of phrases, one that allows the character to have her British sense and some other combination of surprise. Perhaps it is time to tweet the writer of the episode.
It's nonsense , meant to be nonsense, it was making fun of British English for American viewers. Screenwriters often make fun of other cultures for their target audience , to sell the script.
What do you think?