History
Before the Norman Conquest, thou was governed by a fairly simple rule. It did not differ in usage from ye/you; thou addressed a single person, ye more than one.
From French, English acquired the habit of addressing kingss and other aristocrats in the plural. Eventually, this was generalised, as in French, to address any social superior or stranger with a plural pronoun, which was felt to be more polite. In French, it came to pass that tu was intimate, condescending, and to a stranger potentially insulting, while the plural form vous was reserved and formal. In languages that use pronouns this way, it is called the T-V distinction.
Something of this did appear in English. At the trial of Sir Walter Raleigh, Sir Edward Coke, prosecuting for the Crown, reportedly sought to insult Raleigh by saying,
- I thou thee, thou traitor!
here using thou as a verb meaning "to call thou." However, the practice never took root in English the way it did in French.
William Tyndale, seeking to preserve the singular and plural distinctions he found in his Hebrew and Greek originals, consistently used thou for the singular and ye for the plural regardless of the relative status of speaker and listener. By doing so, he probably saved thou from utter obscurity, and gave it an air of solemnity that sharply distinguished it from its French counterpart. Tyndale's usage was imitated in the King James Bible, and remained familiar because of that translation.
William Shakespeare occasionally seems to use thou in the intimate, French style sense, but he is by no means consistent in using the word that way, and friends and lovers call each other ye or you as often as they call each other thou. In Henry IV, Shakespeare has Falstaff mix up the two forms speaking to Prince Henry, the heir apparent and Falstaff's commanding officer, in the same lines of dialogue. It might be said here that the Prince combined the roles of prince and drinking companion:
- PRINCE: Thou art so fat-witted with drinking of old sack, and unbuttoning thee after supper, and sleeping upon benches after noon, that thou hast forgotten to demand that truly which thou wouldest truly know. What a devil hast thou to do with the time of the day? …
- FALSTAFF: Indeed, you come near me now, Hal … And, I prithee, sweet wag, when thou art a king, as God save thy Grace – Majesty, I should say; for grace thou wilt have none -–
Thou had almost gone out of usage entirely in most English dialects by the year