Brittish is the Middle English form of the word British. Middle English was the prevalent form of the English language from the twelfth century to the fifteenth century, but the word Brittish appears on documents up until the eighteenth century.
Most notably, Brittish appears in the Declaration of Independence. Observant readers may note that the Declaration of Independence was written in 1776, over two hundred years after the close of the fifteenth century. Why the double-t then?
By 1776, the English language began to develop independently in England and America. Some words were retained in America much later than they were in England. Certain formalities, such as the double-t in Brittish, may have been kept in the American dialect. This may explain the usage of Brittish in several documents of the time: another example is Rev. William Emerson's account of the deaths at the Battle of Lexington and Concord, "Account of ye. Loss we the Subscribers sustained in ye. 19th of Apr. 1775 by ye. brittish Troops."
Additionally, historian Stephen Lucas notes, "Thoroughly versed in classical oratory and rhetorical theory as well as in the belletristic treatises of his own time, Thomas Jefferson, draftsman of the Declaration, was a diligent student of rhythm, accent, timing, and cadence in discourse." 1 Jefferson's love of language may have inclined him towards the beautiful and formal language of the term Brittish.
Plus, on a more human level, language endures. In the late 20th century, it is not uncommon to hear thee, 'tis and yonder, words that came into use hundreds of years ago, according to Merriam Webster's Dictionary.
Footnotes:
- Stephen E. Lucas, 'The Stylistic Artistry of the Declaration of Independence', 1989.