1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): sign on the dotted line
1. Lit. to indicate one’s agreement or assent by placing one’s signature on a special line provided for that purpose. (The line may be solid or dotted.) • I agreed to the contract, but I haven’t signed on the dotted line yet. • When you have signed on the dotted line, please give me a call. 2. Fig. to indicate one’s agreement to something. • Okay. I agree to your terms. I’ll sign on the dotted line. • He is thinking favorably about going with us to Canada, but he hasn’t signed on the bottom line.
McGrawhill's American Idioms And Phrasal Verbs