This word combo shows that something is supposed to be funny. You now have to laugh. It can also be used for major amounts of tomfoolery by following it with "your mom". ha. ha. ha. ha. ha.
The most overused phrase that every meme tends to begin with. For example, "me when I stub my toe" or " me when my parents are sleeping" etc etc. My theory is this phrase is so unoriginal to the point that it's actually amusing. It's gotten so overused to the point that people are now using this phrase as a meme itself, in an ironic or knowingly absurd sort of way. It's often exaggerated by adding more "me when" to the end.