Basically the same thing as iunno, it is a quickened version of "I do not know", used in both chatting and in informal speech.
It's historymaytrace back to Homer Simpson, who often makes a sound that is in between "iono" and "iunno". (Says a lot about his intelligence, doesn't it?)
PersonMan: So, whatchu up totonight?
TriangleMan: iono. How bout j00?
PersonMan: iono.
TriangleMan: das coo.
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).
Precedes or follows either a gratuitously disrespectful or honest remark, originated in New York City. Used in similar fashion to "no offense" or "to be honest," interchangeably.
Ya mans is wild buns, respectfully. Respectfully he's washed.
After school, The Modells on Jamaica Ave was active. Respectfully.