Hand and Seal
When writing was limited to a few clerks, documents were authenticated by the impression of the hand dipped in ink, and then the seal was duly appended. As dipping the hand in ink was dirty, the impression of the thumb was substituted. We are informed that “scores of old English and French deeds still exist in which such `signatures' appear.” Subsequently the name was written, and this writing was called “the hand.”
Hubert: Here is your hand and seal for what I did. King John: Oh, when the last account `twixt heaven and earth Is to be made, then shall this hand and seal Witness against us to damnation.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894