postscript n.

word-forming element meaning "after," from Latin post "behind, after, afterward," from *pos-ti (source also of Arcadian pos, Doric poti "toward, to, near, close by;" Old Church Slavonic po "behind, after," pozdu "late;" Lithuanian pas "at, by"), from PIE *apo- (source also of Greek apo "from," Latin ab "away from" see apo-).

*skrībh-, Proto-Indo-European root meaning "to cut, separate, sift;" an extended form of root *sker- (1) "to cut."

It forms all or part of: ascribe; ascription; circumscribe; conscript; conscription; describe; description; festschrift; inscribe; inscription; manuscript; postscript; prescribe; prescription; proscribe; sans-serif; scribble; scribe; script; scriptorium; scripture; scrivener; serif; shrift; shrive; subscribe; superscribe; superscript; transcribe; scarification; scarify.

It is the hypothetical source of/evidence for its existence is provided by: Greek skariphasthai "to scratch an outline, sketch;" Latin scribere "to write" (to carve marks in wood, stone, clay, etc.); Lettish skripat "scratch, write;" Old Norse hrifa "scratch."