My first thought was Mississippi as well. Once I found two translations of Mississippi, I decided my thought was wrong.
My source was
They say the Ojibway called it 'Messipi' meaning Big River or 'Mee-zee-see-bee' meaning Father of Waters.
I cecked out the web site you mentioned Finlady and found the Chippewa called it 'mici zibi' meaning Great River.
In a lot of laguages, some words have to be loosely translated. I think that is the case here. All three native words loosely translate into a phrase about the river's size. Thanks to Finlady, I now think the correct answer is Mississippi. Longest, big and great, all really mean the same in this instance.