A harlequinade is the part of a comedy that features the harlequin, an old-fashioned clown. A harlequinade is also any time someone clowns around, like if you put on a fake mustache and imitate your math teacher.
A harlequinade was originally the part of a pantomime that featured the harlequin, a mute character with a mask and diamond-patterned tights, goofing around with a clown. Since those aren’t performed as much now as they were in the 18th and 19th centuries in England, a harlequinade also refers to any time people engage in high-jinx and mayhem. If you wanted to tell someone to get serious while showing off your vocabulary, you could say, "Stop the harlequinade!"