Mid 19th century; earliest use found in The London Medical Gazette. From sub- + sign.
subsign2
/ˈsʌbsʌɪn/
Now rare
verb
In pass. With the name of the signatory as complement. historical in later use.
Origin
Late 16th century (in an earlier sense). From classical Latin subsignāre to write below as a signature, to pledge, to promise solemnly from sub- + signāre.