Old English eow, dative and accusative plural of þu (see thou), objective case of ge, "ye" (see ye), from West Germanic *iuwiz (cf. Old Norse yor, Old Saxon iu, Old Frisian iuwe, Middle Dutch, Dutch u, Old High German iu, iuwih, German euch), from PIE *ju.
Pronunciation 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, originally 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. For a more thorough discussion of this, go here. Through 13c. English also retained a dual pronoun ink "you two; your two selves; each other."
Words for "you" in Japanese include anata (formal, used by a wife when addressing her husband), kimi (intimate, used among friends) or the rougher omae (oh-MAI-aye), used when talking down to someone or among male friend showing their manliness. Dial. you-uns, for you-ones, first noted 1810 in Ohio.
2nd nominative singular personal pronoun, Old English þu, from Proto-Germanic *thu (cf. Old Frisian thu, Middle Dutch and Middle Low German du, Old High German and German du, Old Norse þu, Gothic þu), from PIE *tu-, second person singular pronoun (cf. Latin tu, Irish tu, Welsh ti, Greek su, Lithuanian tu, Old Church Slavonic ty, Sanskrit twa-m).
Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. Philadelphia Quakers). The plural at first was used in addressing superior individuals, later also (to err on the side of propriety) strangers, and ultimately all equals. By c.1450 the use of thou to address inferiors gave it a tinge of insult unless addressed by parents to children, or intimates to one another. Hence the verb meaning "to use 'thou' to a person" (mid-15c.).
Avaunt, caitiff, dost thou thou me! I am come of good kin, I tell thee! ["Hickscorner," c.1530]A brief history of the second person pronoun in English can be found here.
A thousand, esp a thousand dollars; grand: A hundred and fifty thou is business (1867+)