Can anyone tell me what Imao stands for?

There's a good chance that it's not imao, but lmao.

In that case, it means laughing my @ss off

I just wanted to add that the kind of abbreviation that you have listed is referred to as an acronym. Some might call them chat, chat room language, chat room abbreviations, computer jargon, chat room slang, initialisms or just plain chat, etc. There are some pretty comprehensive lists available out there on the web for them Some of those sites are listed below.

Chat Slang and Acronyms used in chat rooms, IM, and email (http://www.web-friend.com/help/lingo/chatslang.html)

Chat Room Acronyms | Telephony and Computer Related Acronyms and Their Meanings | Acronyms Online (http://acronymsonline.com/lists/chat_acronyms.asp)

Chat Room Acronyms - IM Acronym List - Online Acronym Glossary (http://www.real-knowledge.com/chatacro.htm)

Action emoticons (http://www.muller-godschalk.com/acronyms.html)