thank you

The story

1. Middle English · inherited fromthanke yow
2. English todaythank you

From Middle English thanke yow; as with thank God, praise be to God, and praise the Lord, it was originally a present subjunctive form but is usually not parsed that way in current usage. It has also sometimes been parsed as a prodrop form of (present indicative) I thank you, a mechanism that may plausibly coexist with the other one (instances being variable).

First known use, by meaning

• from c. 1400 An expression of gratitude or politeness in response to something done or given.