To talk about what they "originally" meant, we can jump way back to a theoretical language called Proto Indo European (PIE). It is theoretical because we have no direct evidence of the language, but linguists can guess what words might be there by looking at the languages that are related to it. Sometimes they even can guess at the general area where languages developed based on what words are or aren't there. If there is no word for "snow", for example, they might conclude that a language developed in a warmer climate (this is a simplistic explanation from a non-linguist). Certain types of words come up everywhere, like "mother", "father", or "you". Since PIE is a theoretical language, whenever a PIE word is written it is prefixed with a * to indicate that it is theoretical.
Here's a sample of how to say "you" to a single person in a number of languages in the PIE family (you can find more here):
| PIE | English | German | Greek | Latin | Sanskrit | Baltic |
|---|---|---|---|---|---|---|
| *tu | thou/you | du | sú | tū | t(u)vám | ty |
I don't think it would be surprising to find out that whatever came before Proto Indo European used similar words as well. When Middle English switched over to Modern English, it still used "thou" to refer to a single person. Over time, however, "thou" dropped out of usage and English no longer had separate words for the second person singular and plural pronouns and just used "you" for everyone.
Before Modern English came along, however, there was an additional distinction added to "thou" and "you", which is that you were expected to address anyone superior to you as "you", while your superiors would address you as "thou". It would be an insult to refer to a superior as "thou". You can see this reflected in Shakespeare's "Twelfth Night" where Sir Toby recommends says to Sir Andrew with regard to taunting Cesario "If thou thou'st some thrice it shall not be amiss". By the 16th century, "thou" was starting to disappear in London, but that didn't stop the Quakers at that time from always using "thou" to refer to a single person regardless of any social class (with an idea that everyone is equal before God). Quakerism started in the north of England, so "thou" was probably still in use - even today in Yorkshire you might hear someone use "tha" instead of "you". Quakers continued using "thou" long after it had faded from general use, and strangely in America, Quakers eventually switched from "thou" to "thee".
Why is it strange to use "thee" instead of "thou"? "Thee" is used as the object of a sentence, in the same way that you might use "me" instead of "I". You don't say "Me will go to the store", saying "Thee will go to the store" would be the same grammatical "mistake". (I put mistake in quotes because I acknowledge that usages come and go, and while the Quaker usage of "thee" is a departure from the original, it is a correct form as far as that subculture is concerned). So traditionally, you use "thou" where "I" would be correct in a sentence, "thee" for "me", and "thy" for "my".
It may also seem odd that "thou" was used to those considered socially inferior, because today that perception is often reversed. I suspect that this has to do with the Bible. Starting with the Tyndale translation, and through a number of other English translations including the King James Bible, thou/thee/thy was used for the second person singular no matter the social standing. Several 20th-Century bible translations (the Revised Standard Version, the New English Bible, and the New American Standard Bible) opted to keep "thou", but only when referring to God, and "you" everywhere else. I have seen other religious literature, not always Christian, use "thou" in referring to a deity as well. Here's a non-biblical example of using "thou" to refer to a superior:
Other languages have developed a formal-informal usage may or may not reflect a difference in social class. For example, in Spanish you would use "Usted" to refer to a stranger, someone older than you, or someone "superior" to you. In German, you would use "du" with family, close friends, and children, and Sie otherwise (with some exceptions). One of the quirks of Kommissar Keller in the great German crime drama "Der Kommissar" is that he said "du" to his detectives. The Germans have a word for this - "duzen" (pronounced doot-zen), which means to refer to another as "du". As a friendship grows, one might say "wollen wir uns duzen?" (do we want to refer to each other as "du"). Late in the "Der Kommissar" series, one of the detectives moved to another department (he actually moved to another detective show called "Derrick"), and was replaced by his brother, who was also his brother in real life. In this scene, Kommissar Keller tells him:
Ich muß dir gleich etwas sagen. Ich habe vielleicht die etwas schlechte Angewohnheit, meine Mitarbeiter bei der täglichen Arbeit zu duzen. Und das ärgert manche Leute, die finden das nicht richtig und die halten das für autoritär, das ist natürlich vollkommener Quatsch mit Soße. Ich duze euch weil.. ja.. einfach weil ihr mir näher steht als alle anderen und weil es vielleicht ein bißchen zu spät sein dürfte, mich jetzt noch zu ändern. Begreifst du das?
In English it would be:
I must tell you something right now. I may have the bad habit of calling my staff 'du' in the daily work. And that annoys some people, who think it's not right and the think it is authoritarian, which is naturally complete nonsense with gravy. I call you 'du' because.. yes.. simply because you are closer to me than anyone else, and maybe because it's a bit too late to change. Do you understand?
I love the phrase "Quatsch mit Soße" - "nonsense with sauce", although I think it is correct and funnier to translate Soße as "gravy" instead of "sauce" here.
I hope this has been helpful to thee.
