It sounds fancy, but means a casual lover. A fuck buddy. A friend with benefits. Though it can also carry the implication of being an out-of-wedlock lover, as it dates back to a time where having a fuck buddy was almost certainly a sign of married infidelity.
Kith
Means one's friends and other people they are close to that aren't family. Often paired with "kin". Kith and kin. Friends and family.
A paramour is an “other lover”. Para = beside, amour = love. It’s not a casual fuck buddy, it’s your cheating partner. I’m surprised to hear you say it’s unknown as a word these days? Seems like just a normal word to me, albeit one I’m happy to go without using as cheaters suck.
Interesting. The only two references I've ever heard to Paramour are the band and the achievement in Mass Effect. I'm now wondering if the devs of that series knew exactly what it meant (infidelity) because you get the achievement for having any relationship. Maybe it's because you can't remain loyal to your original partner to get it in all three games with one playthrough.