you can joke是什么意思?

you can joke是什么意思?
W: Professor Howl. Have you heard of him?
M: Mh, he does have a good reputation in the political science department
W: And a well deserved one. The same students who fall asleep in discussion groups and seminars fight for front-row seats in his lectures.
M: Oh, no. I hope this isn’t catching
W: You can joke. But it's great to have a professor who's not only interesting but prepared to give up time to students.