The second personal pronoun, in the singular number, denoting the person addressed; thyself; the pronoun which is used in addressing persons in the solemn or poetical style. [1913 Webster
" “In Old English, generally, thou is the language of a lord to a servant, of an equal to an equal, and expresses also companionship, love, permission, defiance, scorn, threatening: whilst ye is the language of a servant to a lord, and of compliment, and further expresses honor, submission, or entreaty.”" Skeat.
" Thou is now sometimes used by the Friends, or Quakers, in familiar discourse, though most of them corruptly say thee instead of thou."
"Art thou he that should come?"