Is it ensure, insure, or assure?
Old English þin, possessive pronoun (originally genitive of þu "thou"), from Proto-Germanic *thinaz (cf. 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 pronomial base *tu-. A brief history of the second person pronoun in English can be found here; see also thou.
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Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. Philadelphia Quakers).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.
The final crisis is here; the unavoidable has come; prepare for the worst: He held her hand fast and said ''This is it, kid'' (1942+)