To sign a document after someone else has signed it, confirming or authorizing their signature.
From counter- + sign (from Latin signum, mark). A legal term developed in medieval and Renaissance times when multiple signatures were needed for official documents.
Countersigning is why contracts often have multiple signature lines—each person countersigning adds a layer of verification and makes forging the document much harder.
Complete word intelligence in one call. Free tier — 50 lookups/day.