Middle English, from Old English eow, dative and accusative plural of þu (see thou) and objective case of ge, "ye" (see ye), from Proto-Germanic *juz-, *iwwiz (source also of Old Norse yor, Old Saxon iu, Old Frisian iuwe, Middle Dutch, Dutch u, Old High German iu, iuwih, German euch), from PIE *yu, second person (plural) pronoun.

The pronunciations of you and the nominative form ye gradually merged from 14c.; the distinction between them passed out of general usage by 1600.

Widespread use of French in England after 12c. gave English you the same association as French vous, and it began to drive out singular nominative thou, first as a sign of respect (similar to the "royal we") when addressing superiors, then equals and strangers, and ultimately (by c. 1575) becoming the general form of address.

You know as a parenthetical filler is from 1712, but it has roots in 14c.; as as a euphemism for a thing or situation unmentionable it is from 1867. You never know as a response to something unexpected is attested from 1924.

Phrase you-know-what in place of something deliberately not named is by c. 1600 (1540s as you wot what). You-know-who (or whom) for a person it is thought best not to name (but implying the hearer knows) is by 1766. You never know as a response to something unexpected is attested from 1924.

Through 13c. English also retained a dual pronoun ink "you two; your two selves; each other."

as an adjectival phrase in reference to supercilious sanctimony attested by 1841. The text is in Isaiah lxv.5. See holy (v.) + thou.

colloquial for "the French language," 1754, from French parlez-vous (français?) "do you speak (French?)" From parlez, second person plural of parler "to speak" (see parley (n.)) + vous, from Latin vos, plural of tu "thou" (see thou). Also used as a verb, "to speak French." It got another boost in U.S. after World War I, along with other mangled French terms brought home by the doughboys, such as san fairy ann, a jocular expression of indifference, representing French ça ne fait rien "it does not matter."

Middle English the, from Old English þe (accusative and dative singular cases 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).

In Middle English, plural forms (see you) began to be used in all cases, at first as a sign of respect to superiors (a parent, king, victor, God, Christ, the Virgin), also in direct address by God; then as a courtesy to a social equal (also a spouse, lover).

By 17c. the singular forms had come to represent familiarity and lack of status, and they fell from use except in dialects. Lancashire north of the Rossendale Forest and Yorkshire formerly were noted for use of the singular second person pronouns tha (nom.) and thee (acc.).

The Quakers also retained the familiar forms, for religious reasons (Christian equality of persons), but it also was justified as grammatically correct. Hence the verb thee "to use the pronoun 'thee' to someone," by 1662, in connection with Quakerism.

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]

Compare French tutoyer "treat as an intimate, address familiarly," from singular pronouns to, toi, te, used instead of plural vous.

"of thee," Old English þin, possessive pronoun (originally genitive of þu "thou"), from Proto-Germanic *thinaz (source also of Old Frisian, Old Saxon thin, Middle Dutch dijn, Old High German din, German dein, Old Norse þin), from PIE *t(w)eino-, suffixed form of second person singular pronominal base *tu-. See also thou.

"of or pertaining to thee," possessive pronoun of the second person singular, late 12c., reduced form of þin (see thine), until 15c. used only before consonants except -h-. Compare mine/my, an/a. It is the possessive of thou, in modern use only in solemn and grave style.

"you," in addressing more than one, Old English ge, nominative plural of 2nd person pronoun þu (see thou).

Sometimes in Middle English of individuals: occasionally derisively or as a social insult, but often of a social superior, parent, God, Christ, Mary, by a man of his lady or a woman of her husband, also in politeness to a stranger and of anyone from whom a favor is desired or has been received.

Germanic cognates include Old Frisian ji, Old Saxon gi, Middle Dutch ghi, Dutch gij. Outside Germanic, cognates include Lithuanian jūs, Sanskrit yuyam, Avestan yuzem, Greek hymeis.

Altered, by influence of we, from an earlier form similar to Gothic jus "you" (plural). The -r- in Old Norse er, German ihr probably is likewise by influence of the 1st person plural pronouns in those languages (Old Norse ver, German wir).

For more, see you.

The confusion of the two forms, and the use of you as nom. began early mod. E., and is conspicuous in the Elizabethan dramas. In the authorized version the Bible (1611), in which many usages regarded as archaisms were purposely retained, the distinction between ye, nom., and you, obj., is carefully preserved. [Century Dictionary]