2nd nominative singular personal pronoun, O.E. þu, from P.Gmc. *thu (cf. O.Fris. thu, M.Du., M.L.G. du, O.H.G., Ger. du, O.N. þu, Goth. þu), from PIE *tu-, second person singular pronoun (cf. L. tu, Ir. tu, Welsh ti, Gk. su, Lith. tu, O.C.S. ty, Skt. twa-m). Superseded in M.E. by plural
(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" (c.1440).
"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 Eng. can be found here