I've never lived in the mid-west or the New England region of the USA. Maybe it's a regional usage (I've lived in Florida, Texas, California, Colorado, Utah, Oregon, and Washington). I'm not sure where I picked up my usage from. My dad is from Colorado and my mom from California. Maybe I picked it up from one of them ;-)
1. (of a person) surprised and confused so much that they are unsure how to react. "he would be completely nonplussed and embarrassed at the idea"
2. INFORMAL•NORTH AMERICAN (of a person) not disconcerted; unperturbed.