1610s, abbreviation of Latin post scriptum (see postscript).
Entries linking to p.s.
postscript n.
"an addition made to a written or printed composition," especially a paragraph added to a letter which already has been concluded and signed by the writer, 1550s, from Latin post scriptum "written after" (compare postscribere "to write after"), from post "after" (see post-) + neuter past participle of scribere "to write" (from PIE root *skribh- "to cut").