To write positive things about people with your own first name on urban dictionary. In most, however not all, cases a way of making up for personal insecurities.
A less popular form of Katieing is to write opinions about other people you know.
Whether or not the statements written about the concerning person are true or not, is irrelevant.
Jimmy: Simon has been Katieing like crazy man, he writes he has a huge c*ck!
Summer: To be fair, he does...
Internet meme (like horsemanning or planking) where one person lies on their stomach between a standing person's legs posing for a photo. This usually occurs for vacation pictures.
Sneak into your parents' room on their anniversary night to see an example of katieing.