From Longman Dictionary of Contemporary Englishsomething is no jokesomething is no jokeSERIOUS SITUATIONused to emphasize that a situation is serious or that someone really means what they say The risk he’s taking is no joke. It’s no joke bringing up a child on your own. → joke
Examples from the Corpus
something is no joke• These bills are no joke.• Pneumonia is no joke, you know.• Now that is no joke because one of these big old saws can weigh the best part of half a ton.• Especially where alcohol is concerned, this is no joke at all-as the men from Rutgers ably demonstrated in the 1970s.• This is no joke, Guy.• This is no joke, mister, it's for real.• The bike fell on him, tearing his Achilles tendon, which is no joke at all. 