late 14c., proscripcioun, "decree of condemnation, outlawry, sentence of exile, the dooming of a citizen to death as a public enemy and confiscation of his goods," from Latin proscriptionem (nominative proscriptio) "a public notice (of sale); proscription, outlawry, confiscation," noun of action from past-participle stem of proscribere "publish in writing" (see proscribe).
late 14c., prescripcioun, in law, "a title or right acquired through long use or uninterrupted possession," from Old French prescription (13c.) and directly from Latin praescriptionem (nominative praescriptio) "a writing before, order, direction," noun of action from past participle stem of praescribere "write before, prefix in writing; ordain, determine in advance," from prae "before" (see pre-) + scribere "to write" (from PIE root *skribh- "to cut").
Meaning "act of establishing by rules" is from 1540s. The medical sense of "written directions from a doctor of the medicines or remedies to be used by a patient and the manner of using them" is recorded by 1570s. The word has been confused with proscription at least since c. 1400.