Why - and when - did "thou" disappear?

In the Early Modern English Period, "you" spread more and more, at the expense of "thou": In cases of doubt, it was safer to say "you", as it was the more polite form. Furthermore, the usage of "you" spread down in society because lower classes, who aspired social advancement, imitated the custom of higher classes, who used the reciprocal "you".

According to Barber, "by 1600, you was the normal unmarked form of the singular pronoun in all classes with any pretensions to politeness, while thou was the form which carried special implications. [...] In the course of the 17th century, you steadily displaces thou in educated usage" (Barber, 210).

Finkensteadt notes that the idea of tolerance and general respect for one's fellow men has contributed to the disappearance of "thou" because this pronoun was considered as address for inferiors. This aspect is supported by the fact that the American settlers in the 17th and 18th century barely used "thou" (cf. Finkensteadt, 225).

Nevertheless, even in the middle of the 17th century, people were still very aware of the fact that that "thou" had several special implications. The use of "thou" in "inappropriate situations" could provoke annoyance or "shocks" (cf. Barber, 211). The reaction towards the Quakers, who insisted on the distinction between "thou" and "you" as a marker of number, is a good example for one of the consequences which the decision for "thou" could have.

In his A Grammar of the English tongue Samuel Johnson (1709-1784) wrote "You is commonly used in modern writers for ye, particularly in the language of ceremony, where the second person plural is used for the second person singular". This could imply that "thou" was still used in contexts which did not belong to the "language of ceremony" in the 18th century. In contrast to this the Merriam Webster Dictionary of English Usage says that "for most speakers of southern British English thou had fallen out of everyday use around 1650 (quoted after Wikipedia entry dealing with "thou".