"You __ be serious!" (4)

I believe the answer is:

cant

(Other definitions for cant that I've seen before include "Hypocritical talk; slope" , "Lacks ability" , "Hypocritical speech" , "Insincere talk; slope" , "Hypocritical talk sounds unable" .)

I've seen this clue in the Newsday.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.