Imaoo means laughing your a** off.
Imaoo seems to be a dumba55 combination if LMAO - "laugh my a55 off" and IMAO, which means "In my arrogant opinion" - unless someone knows better.
lmao is internet shorthand for "laughing my a** off." The extra o's mean the laughter is continuing.
I'm assing off
Lame