This is currently translated “That’s not on!” but I think something like: “We didn’t bet on that!” or something of the sort might be better.
This sentence is to tell someone that he/she is not playing by the rules.
From my point of view it should not be in fastpath or common words, because you only use it if you know someone really good.
That makes sense. A tricky thing about the fastpath and common words is that they are based on an algorithm that selects sentences from Tatoeba based on word frequencies, and as such they don’t take into account the frequency of use of a particular idiom or grammatical construction. This is where it would be great for Clozemaster to have people reviewing them and then flag a particular sentence to be passed over and the algorithm could grab the next-best sentence, and hopefully it would be an improvement?