see under no joke.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for laughing matter
There was a hideous jocularity in some of these imperial proceedings, which, however, was no laughing-matter at the time.The Eighteen Christian Centuries