No laughing matter
Not a subject for levity.
Like 'no holds barred' and 'no room to swing a cat', 'no laughing matter' is almost always expressed as a negative. Comic situations aren't usually referred to as laughing matters. That wasn't always the case - 'laughing matter' was a commonplace idiom before anyone ever thought to make a negative phrase out of it.
The phrase sounds as though it might be of quite recent coinage but in fact it has been known since Tudor days, as in this example where the Bishop of Worcester, Hugh Latimer published a tirade against the selling of religious favours - The seconde sermon preached before the kynges maiestie, 1549:
These sellers of offices shew that they beleue that there is neyther hell nor heauen. It is taken for a laughynge matter.
Edward V, who was king at the time, couldn't have been overly impressed as the selling of favours continued for many years after this sermon was preached.
Latimer was nothing if not persistent and the first instance of 'no laughing matter' is found in John Foxe's reports of his continuing campaign against religious corruption - Actes and monuments touching matters of the Church, 1563:
Then the audience laughed againe, and maister Latimer spake vnto them saying: why my maisters, this is no laughyng matter. I aunsweare vpon life and death.
Latimer was devout and, if contemporary reports are to be believed, of a rather dour disposition - which is appropriate for the chief instigator of a phrase like 'no laughing matter'. His parents considered that he had a "ready, prompt, and sharp wit" but by 'wit' they meant 'intellect' rather than 'humour' and "purposed to train him up in erudition".