no joke

Also, no laughing matter. A serious issue, as in Missing the last flight out was no joke, or This outbreak of flu is no laughing matter. The first term dates from about 1800, and the variant from the second half of the 1500s.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.