Translation:You are not able to be serious.
As a longtime tennis fan (and I do mean longtime - I was around for, and remember, John McEnroe's glory days), I was actually just putting this sentence together in my head, and thinking of it as a potential sample sentence entry for the course, when I looked down at my screen ... and voilà! There it was, right in front of me. Nice job on the reference, and the anticipation! ;-)
True. The exact McEnroe quote was "You cannot be serious," which differs only in the contraction in the English translation. But as you say, it's generic enough to mean different things to different people.
Actually, isn't this the Airplane quote where one character says "Surely you can't be serious!" and Leslie Nielsen' character replies, "I am serious. And don't call me Shirley"?