A composite word using common (and not so common) internet acronyms.
Here is a break down:
Rofl : Roll On Floor Laughing
MAO : My Ass Off
UIVSM : Until I Violently Soiled Myself
Used to show that you found something so incredibly funny that you lost control of your anal sphincter in an extremely violent way.
First seen on LED boards at a Blue Man Group stage performance.
Friend: *Insert particularly hilarious joke*
You: Oh man I just Roflmaouivsm!!