You've got it sorta backwards, at least as far as English is concerned. "Thou" and "thee" were the descendents of Old English "þū" and "þē", respectively, and

You've got it sorta backwards, at least as far as English is concerned. "Thou" and "thee" were the descendents of Old English "þū" and "þē", respectively, and were originally the nominative and dative[1] forms of the singular second-person pronoun. "Ye" and "you" (< OE "ġē" and "ēow") were the corresponding forms of the plural second-person pronoun. In the High Middle Ages, English adopted the "tu-vous distinction", named after the French pronouns corresponding to "thou-you", wherein social superiors were always addressed in the plural (cf. "the royal we"). "Thou" fell out of use because people did not want to risk offending anyone by implying the other was their social inferior, wherefore they began addressing everyone as a superior. When Shakespeare writes "I thou thee!", it's not to ironically mock, but rather to explicitly disparage, by asserting that the recipient is inferior, unworthy of being ye'd. Of course, the vacuum created by the loss of a productive singular was later filled by constructions like "y'all", "you guys", "yous", &c. The same phenomenon occurred in Dutch, where the singular "du"/"di" was replaced by the plural (and therefore formal) "gi/u" during the Middle Dutch period, and the plural later filled by "jullie", a contraction of "je lui", meaning "you folks". [1]: In late Old English, also the accusative, replacing "þec". From Middle English, the oblique (the sole objective case).