A neologism coined by Röandil posted on 2023-02-28 in the Vinyë Lambengolmor Discord Server (VLDS), derived from a primitive instrumental form *rinkumā base on √RIK “try, strive, endeavor” (comparable to ᴹQ. sanguma for similar derivation), thus more literally “a thing striven/endeavored for”. Röandil also suggested rinqua as the result of the loss of the intervening u after which [km] became [kw], but the combination nkm seems very unlikely to me and I prefer rincuma.