you pron.

2nd nominative singular personal pronoun, Old English þu, from Proto-Germanic *thu (source also of 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 (source also of 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Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. early Quakers).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.

Old English ge, nominative plural of 2nd person pronoun þu (see thou); cognate with Old Frisian ji, Old Saxon gi, Middle Dutch ghi, Dutch gij. Cognate with Lithuanian jūs, Sanskrit yuyam, Avestan yuzem, Greek hymeis.

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