![By Government of the United Kingdom. (Transferred from en.wikipedia.) [Public domain], via Wikimedia Commons By Government of the United Kingdom. (Transferred from en.wikipedia.) [Public domain], via Wikimedia Commons](https://upload.wikimedia.org/wikipedia/commons/b/bf/Letters_Patent_Australia.jpg)
Click for a larger image
The medical terms [patent] and [patency] use the root term [-pat-] which arises from the Latin word [patentum] and its nominative form [patens], meaning "open". It refers to a structure that is open allowing for free flow.
The suffixes [-ent] and [-cy] are abstract suffixes (similar to adjectival suffixes) that are used as "pertaining to", or simply ignored.
Note of interest: An initial look at this term used in medicine clashes with the legal meaning of [patent] which is defined as a "a government authority or license conferring a right or title for a set period, especially the sole right to exclude others from making, using, or selling an invention" (Google search). Fact is, they are similar and arise from the same origin.
In medieval times letters meant to be read by a single individual were sealed, but government letters, edicts or resolutions meant to be read by many or be the law of the land where written as [litteræ patentes] (medieval Latin) which can be loosely translated as "an open letter". These "Letters of Patent" where written in parchment with seals and looked impressive! The term later was reduced to "patent", thus the double meaning for this word both in the medical and legal arenas.
Note: The links to Google Translate include an icon that will allow you to hear the pronunciation of the word.
Clicking on the image will take you to an example of a patent (Australia)