To Jacknife: When you burst out laughing, so hard and so suddenly,
you involuntarily fold in half at the waist.
Usually accompanied by a large exhale of breath.

Here’s the official entry on UrbanDictionary.com

To Jacknife – pass it on!

PS – I got the good news email on a #ThursdayNight.  Further proof it really is the best night of the week.