WORD FORMATION RESOURCE
Middle English: from Old French signe (noun), signer (verb), from Latin signum ‘mark, token’. Sign comes via Old French from Latin signum ‘mark, token’. From the same source come signal (Late Middle English), significant (late 16th century), signet (Late Middle English) ‘small seal’ with which you make your mark, and mid 16th-century signature, which was first used as a Scots legal term for a document presented by a writer for royal approval and seal. Resign (Late Middle English) is from Latin resignare ‘unseal, cancel’.
Rather than signing their name, people formerly stamped a personal seal in wax on a completed letter or other document. The expressions put the seal on, ‘to put the finishing touch to something’, and set your seal to, ‘to mark something with your own distinctive character’, both derive from this. To seal something off reflects the use of seals to check that something has not been opened or disturbed. In these and related uses, seal goes back to Latin sigillum ‘small picture’, from signum ‘a sign’, the source of design (late 16th century), designate (mid 17th century), ensign (Late Middle English), insignia (mid 17th century), sign (Middle English), signal (Late Middle English), scarlet, and numerous other English words. This seal dates from Middle English. The name of the animal seal derives from Old English seolh, the source also of the selkie or silkie (mid 16th century), the mysterious seal woman of folklore.