kitten (intransitive verb)

1. To laugh so hard Pepsi (or whatever you're drinking) comes out your nose.
2. To laugh suddenly and uncontrollably.

kitten, a founder of the UseNet newsgroup alt.callahans, responded to particularly good, funny posts with the comment "I laughed so hard Pepsi came out my nose." Laughing like that has come to be known as kittening.

Note that when used in this sense, "kitten" or its variant is always lowercase, regardless of its placement in a sentence.