Merriam-Webster's Collegiate® Dictionary, 11th Edition
THOU
I. pro noun
Etymology: Middle English, from Old English thū; akin to Old High German dū thou, Latin tu, Greek sy Date: before 12th century the one addressed ,
II. transitive verb
Date: 15th century
to address as thou ,
III. noun (plural thou)
Etymology: short for thousand
Date: 1867
a thousand of something (as dollars)