How would Shakespeare have said "Thank you"? Can't decide if it is thee or thou, since it isn't really a sentence.
On a quick look through the concordance, it appears that Shakespeare rarely wrote Thank you and never Thank thee without a subject. He often wrote I thank you and we thank you (and forms such as to thank thee and shall thank thee); but for a shorter form without a subject he usually used Thanks.
Thank thou would be ungrammatical, unless it was followed by an object, and would then be a command: Thank thou the king!. But there do not seem to be any instances of thank thou in his writings.
Firstly you should realise that the English language was in a state of flux during Shakespeare's time. You will find inconsistencies. Shakespeare's English was not Old English -- it was Early Modern English.
- "Thank you" as used these days is an abbreviation of "I thank you". Online Etymology Dictionary
- Plural form: The modern 'you' is used for both singular and plural. In Shakespeare's day there was a distinction. For example it would make no sense to say "I thank thee" to a group of people. Instead you would have to say "I thank ye" (familiar form) or "I thank you" (polite form).
MACBETH: I thank you, gentlemen. (polite) Macbeth Act I, scene III
KING HENRY VIII: My noble gossips ... I thank ye heartily; so shall this lady. (A king isn't obliged to be polite to his subjects, especially when he is insulting them!) King Henry VIII Act V, scene V
- Singular form: It is possible to find "I thank you" and "I thank thee" in Shakespeare when spoken to an individual. The explanation is that the plural is used as a sign of respect to an elder or superior. In modern English this respectful form is the only one to survive.
WS2 has given an example of "I thank thee".
CLAUDIO: I thank you, good friend Lucio. (Claudio doesn't have to be so polite to his friend. It's a choice. He is adding a note of respect.) Measure For Measure Act I, scene II
- "Thank thou" is possible for the reason stated by Colin Fine. It would not be a complete utterance and 'thou' would be the subject.
According to the Online Etymology Dictionary, "thanks" is a shortened form of "I give you thanks". The "you" in that phrase is the indirect object of the verb. Therefore, you should use the objective form "thee" rather than the subjective form "thou".
On Wikipedia there is an article which might help: https://en.wikipedia.org/wiki/Thou#Declension
In the article it states that 'thou' is the nominative form and 'thee' is the oblique (accusative?) form. I think that if you are giving thanks to a person then the oblique form is the correct form.
So my understanding would be that 'I thank thee' is the proper formulation.