(Internet slang) secret personal account (on social media, especially Twitter and Instagram (finsta), as opposed to one's openly used account)

back door

underhanded dealings

underbelly of society

(literary, rare) here

(painting) the application of silk as a screen on metal foil, or the metal foil in that application

hardcore pornography; 'underground' pornography showing uncensored genitals. Technically illegal in Japan.

sole of the foot

An underhanded trick. / A lifehack.

passion (song themed around the Passion of Christ)