Thou
Old English thu, of Germanic origin; related to German du, from an Indo-European root shared by Latin tu .
wiktionary
From Middle English thou, tho, thogh, thoue, thouȝ, thow, thowe, tou, towe, thu, thue, thugh, tu, you(Northern England), ðhu, þeou, þeu, þou(the latter three early Southwest England), from Old English þū, [1] from Proto-West Germanic *þū, from Proto-Germanic *þū(“you (singular), thou”), from Proto-Indo-European *túh₂(“you, thou”).
From Late Middle English thouen, theu, thew, thou, thowe, thowen, thui, thuy(“to address (a person) with thou, particularly in a contemptuous or polite manner”), from the pronoun thou: see etymology 1 above. [3]
Short for thou(sandth). [4]
Short for thou(sand). [4]
A misspelling of though.
etymonline
thou (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Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. early Quakers).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.
