See quotation.

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1852.  So well were the people of New Jersey known … in the time of the Revolutionary war, that the term applied to them was “Jersey Blue,” synonymous of truth and fidelity, and even to this day is the same term applied to the inhabitants of that State.—Mr. Skelton of N.J., House of Repr., Aug. 12: Cong. Globe, p. 967, App.

2