Agree formally or fully, as in The deal is just about fixed; all they have to do is sign on the dotted line. This idiom refers to the broken line traditionally appearing at the bottom of a legal document, indicating the place for one's signature. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.