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 history may
trace 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 to tonight?
TriangleMan: iono. How bout
j00?
PersonMan: iono.
TriangleMan:
das coo.