1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): no joke
a serious matter. • It’s no joke when you miss the last train. • It’s certainly no joke when you have to walk home.
McGrawhill's American Idioms And Phrasal Verbs