If I had to guess, the missing adjective is probably
ビッチ which gets romanized to a certain 5-letter English word that means differently as Japanese slang vs the original English meaning.
Maybe Umimi should've gone for it, just so that Riku can finally forget about Shizuku and Takt's mind games. Of course he'd have other problems to worry about (and maybe have to take responsibility too), but perhaps finally getting rid of Shizuku and Takt ends up being a net benefit in his case...