Old English þe (accusative and dative singular of þu "thou"), from Proto-Germanic *theke (source also of Old Frisian thi, Middle Dutch di, Old High German dih, German dich, Old Norse þik, Norwegian deg, Gothic þuk), from PIE *tege-, accusative of root *tu-, second person singular pronoun (see thou). The verb meaning "to use the pronoun 'thee' to someone" is recorded from 1662, in connection with the rise of Quakerism.
In Middle English, people began to use plural forms in all cases, at first as a sign of respect to superiors, then as a courtesy to equals. By the 1600s, the singular forms had come to represent familiarity and lack of status, and fell from use except in the case of a few dialects, notably in the north of England. People in Lancashire north of the Rossendale Forest and Yorkshire formerly were noted for use of the singular second person pronouns tha (nom.) and thee (acc.). For religious reasons (Christian equality of persons, but also justified as grammatically correct), the Quakers also retained the familiar forms.
Thou and Thee was a sore cut to proud flesh and them that sought self-honour, who, though they would say it to God and Christ, could not endure to have it said to themselves. So that we were often beaten and abused, and sometimes in danger of our lives, for using those words to some proud men, who would say, "What! you ill-bred clown, do you Thou me?" as though Christian breeding consisted in saying You to one; which is contrary to all their grammar and teaching books, by which they instructed their youth. [George Fox's journal, 1661]
While the Quakers originally adopted "thee" and "thou" on account of their grammatical correctness, they soon fell into the careless habit of using "thee," the objective, instead of "thou," the nominative. Common illustrations are: "How does thee do?" or "Will thee," etc. [George Fox Tucker, "A Quaker Home," Boston, 1891]