When a fat person has a wet dream over the varity of food offered at McDonalds.
This 'McCreamy' can also occur at any time of the day, normally when a fat person has entered a McDonalds or has seen anything that reminds them of the varity of food offered.
Guy 1. dude whats on that fat guys leg?
Guy 2. Oh my god man, he McCreamy'd himself!