patent (n.) Look up patent at Dictionary.com
late 14c., "open letter or document from some authority," shortened form of Anglo-French lettre patent (also in Medieval Latin (litteræ) patentes), literally "open letter" (late 13c.), from Old French patente (see patent (adj.).
The Letters Patent were ... written upon open sheets of parchment, with the Great Seal pendent at the bottom ... [while] the 'Litteræ Clausæ,' or Letters Close, ... being of a more private nature, and addressed to one or two individuals only, were closed or folded up and sealed on the outside. [S.R. Scargill-Bird, "A Guide to the Principal Classes of Documents at the Public Record Office," 1891]
Meaning "a license covering an invention" is from 1580s.
patent (v.) Look up patent at Dictionary.com
"to obtain right to land," 1670s, from patent (n.). The meaning "copyright an invention" is first recorded 1822, from earlier meaning "obtain exclusive right or monopoly" (1789), a privilege granted by the Crown via letters patent. Related: Patented; patenting.
patent (adj.) Look up patent at Dictionary.com
late 14c., in letters patent, literally "open letter," from Old French patente, from Latin patentum (nominative patens) "open, lying open," present participle of patere "lie open, be open," from PIE *pete- "to spread" (see pace (n.)). Sense of "open to view, plain, clear" is first recorded c.1500. Related: Patently.