1781, in reference to words or phrases used in North America and distinct from British use, coined by John Witherspoon, president of Princeton College, from American + -ism. (American English "English language as spoken in the United States" is first recorded 1806, in Webster.) Americanism in the sense "attachment to or preference for the U.S." is attested from 1797, first found in the writings of Thomas Jefferson.
I have been not a little disappointed, and made suspicious of my own judgment, on seeing the Edinburgh Reviews, the ablest critics of the age, set their faces against the introduction of new words into the English language; they are particularly apprehensive that the writers of the United States will adulterate it. Certainly so great growing a population, spread over such an extent of country, with such a variety of climates, of productions, of arts, must enlarge their language, to make it answer its purpose of expressing all ideas, the new as well as the old. [Jefferson to John Waldo, Aug. 16, 1813]
Jefferson, as if disposed to assail the sovereignty of the English tongue as well as the sovereignty of the English sword, never hesitated to coin a word when it suited his purposes so to do; and though many of his brood are questionable on the ground of analogy and as intermixing languages; yet they were expressive, and became familiar. [Hugh Blair Grigsby, "The Virginia Convention of 1776," 1855]