'Knuist' is normally 'fist'. If it is between two (teenage) male friends, I imagine it may be something like hitting each other's fist, as happens in some hiphop/rap cultures.

I would find it difficult to translate this gesture into English, and would be inclined to go for something like 'Take care' or 'Catch you later' or 'Keep it up'. Perhaps 'Respect' would come closest.

Hope this helps but it would be good to know a bit more about the content of the letter to see if my guess is correct.

Kind regards,

Ron