A Couple of Definitions of "Rarepair"

[beetle]

According to Fanlore:

A rarepair is a term for a pairing or ship that is rare in a fandom. It is not featured in many fanworks and makes up a only small proportion of the overall total. A rare pair may be canon, but is often not. Some pairings may be shipped by a lot of fans, but be uncommon in fanworks. A rarepair may be a fan's OTP, or just shipped on the side.Reasons for a pairing being uncommon include a lack of screentime or interaction in canon, one or both characters being unpopular in fandom, being a crossover ship, featuring an unpopular relationship dynamic (for example, age difference or incest), or the fandom base itself may be made up of people who prefer a different kind of ship – for example, a rare het pairing in a m/m slash-heavy fandom.

According to UrbanDictionary:

A ship or pairing that not many people ship or is very rare to find fanfiction and fanart of. Typically happens within small fandoms or with minor characters. I ship See-More and Billy Numerous from Teen Titans but it's such a rarepair. I like to think that everyone has at least one rarepair.


Feel free to ask for clarification or put it in your own words. But I think most of Fandom's got a good idea of the difference between "popular" and "rare."