In English this means 'to put up a brave front' or 'to put a good face on the matter'. In German it is 'Gute Miene zum bösen Spiel machen'.
Just in case somebody has the same problems understanding this as I had.
It means to keep a straight face. Close but no cigar, Mr. Feynman!. ;-)
...which would then make it "keine Miene verziehen" in German.