I heard somebody say that a database could be sorted by relevancy, which grated on my ears something hijjus. The word is relevance! It’s right there on the screen! There’s no need to add the extra syllable!
Alas, I have the unfortunate tendency, when grousing about usage, to look stuff up, and yeah, relevancy has been in use as a noun form of relevant for as long as relevance has. It’s legit—the OED says that it’s less common than relevance, but less common ain’t necessarily wrong.
The other thing that surprised me was that relevant, in the general sense that we use it today, derives from a legal term, and specifically a legal term from Scotland. That is, at one point (that is, in the early 17th century) if you said that something was relevant to something else, you were talking about a legal case in Scotland, or making an analogy to one. It doesn’t get commonly used in a general non-legal sense until around 1800 or so. Before that? We seem to have pertinent, which is still in use, and to the purpose, which is quaint but still usable, and a bunch of obsolete words or uses such as appliable and incident and relative. Evidently germane is a Shakespearean innovation, deriving from cousin-german , that is, first cousin, and brother-german, full brother, and the like, through the close connection to get to the sense of relevance.
The opposite, which seems important, has of course such structures as irrelevant and unpertinent (also impertinent as well as exorbitent, which are both obsolete in that sense) but also—and this is lovely, innit—peregrine.