You cannot unread what was already read but with hold mode you can have a two-dimensional view of what you type and edit it at any point that wasn't read yet, that means since you started hold mode.

> Even when there are newlines in the output text, 9term will not honor reads if the window is in hold mode [...] Some programs automatically turn on hold mode to simplify the editing of multi-line text; type ESC when done to allow mail to read the text.