According to the Dictionary of American Family Names quoted on
ancestry.com, it is Polish.
"Last Name: Iwanski
1. Polish (Iwan'ski): habitational name for someone from Iwanie
(now Iwonie) in Sieradz voivodeship, a place named from the personal
Dictionary of American Family Names, Oxford University Press, ISBN 0-19-508137-4"
By coincidence, Edmund Iwanski, is president of the Polish
Genealogical Society of America.
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iwanski poland OR polish