common origin for both Newfoundland use and the person-of-Italian-origin-derogatory use.

I think we've got two independent origin words that just happen to be spelled the same. The etymology given in the Newfie Dictionary looks OK to me and is of an era with lots of similar type words. See WAVES, WAC, WRENs, SPARs, etc. All these, I believe (tsuwm could veto me here) came from the '40s.