countersign noun 2verb 2forms 3 noun2 •A second signature added to a document to affirm the validity of the signature of the first person. •The response to a sign or signal.Only those who give the countersign may enter the Masonic Lodge. verb2 •To add a second signature to a document, affirming the validity of the signature of another person. •To sign on the opposite side of (a document). formscountersigned · countersigning · countersigns