message-make-forward-subject-function

message-make-forward-subject-function is a variable defined in `message.el'.
Its value is

message-forward-subject-name-subject

• This variable may be risky if used as a file-local variable.

Documentation:
*List of functions called to generate subject headers for forwarded messages.
The subject generated by the previous function is passed into each
successive function.

The provided functions are:

* `message-forward-subject-author-subject' Source of article (author or
newsgroup), in brackets followed by the subject
* `message-forward-subject-name-subject' Source of article (name of author
or newsgroup), in brackets followed by the subject
* `message-forward-subject-fwd' Subject of article with 'Fwd:' prepended
to it.

You can customize this variable.