The word patent was first used in English in Chaucer's Canterbury Tales, when it denoted a type of open document conferring authority (that's why patently means "openly"). This was taken from Anglo-French phrase lettre patent, meaning "open letter". The relevant part of that comes from Latin patens, a participle for openness, and that in turn is from patere, which could mean "to be open" or "to lie open". Through Proto-Italic, patere is reconstructed as deriving from the Proto-Indo-European root peth, meaning "to fly". The phrase patent law was first used in 1817, and the definition of "sole right to produce something" is from 1558. According to Google NGrams, usage of the word patent in literature over time has remained constant since a spike in the 1750s, but Google Trends has shown it as decreasing over the last 15 years.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.