In Old English "thou" is singular, "you" is plural.
As noted in Wikipedia:
> As William Tyndale translated the Bible into English in the early 16th century, he preserved the singular and plural distinctions that he found in his Hebrew and Greek originals. He used thou for the singular and ye for the plural regardless of the relative status of the speaker and the addressee. Tyndale's usage was standard for the period and mirrored that found in the earlier Wycliffe's Bible and the later King James Bible.
Later, presumably due to French influence, "thou" became informal and "you" formal.
Finally "thou" was dropped from everyday speech, though it still shows up in various old phrases.
As noted in Wikipedia:
> As William Tyndale translated the Bible into English in the early 16th century, he preserved the singular and plural distinctions that he found in his Hebrew and Greek originals. He used thou for the singular and ye for the plural regardless of the relative status of the speaker and the addressee. Tyndale's usage was standard for the period and mirrored that found in the earlier Wycliffe's Bible and the later King James Bible.
Later, presumably due to French influence, "thou" became informal and "you" formal.
Finally "thou" was dropped from everyday speech, though it still shows up in various old phrases.