The phrase 'signed as a deed' is yet to become the part of a standard dictionary and so it is open to interpretation. The most probable meaning that can be figured out from the phrase is to convey the idea of telling something or writing something with determination and there is no possibility of getting back from what has been said.
The meaning can be figured out by finding out the separate meanings of the words 'signed' and 'deed'. The word signed is used to put your signature on something and after putting your signature you are bound to do what you have written. Deed is a word that is used to make a formal contract for something. So, when you are writing the phrase 'signed as a deed' it probably conveys the idea of to be bound to something from which there is no scope to get back.
The meaning can be figured out by finding out the separate meanings of the words 'signed' and 'deed'. The word signed is used to put your signature on something and after putting your signature you are bound to do what you have written. Deed is a word that is used to make a formal contract for something. So, when you are writing the phrase 'signed as a deed' it probably conveys the idea of to be bound to something from which there is no scope to get back.
