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1.
Short for the Infinite-On-Off-Syndrome. This annoying bug sometimes happen in Facebook where the chat system turns on and off at a rapid rate.
A: Hello
B: Sup (send as message?)
A: -offline-
A: LOL
B: WTF? (send as message?)
A: -offline-
A: I got the IOOS :P
B: =.=
by WhoAmI?IamBartMan December 15, 2010
 
2.
IOO
In Our Opinion. The plural form of IMO.
"Is Allan the best dressed man in the world?"

"Yes, IOO"
by SexyJosem July 18, 2012
 
3.
ioo
I don't know.

Instant messaging slang. Usually follows a question.
<Chris> What time do you want to leave?
<Tim> ioo
by Johnno31 August 07, 2006