(cryptography) secret key

secret police

secret base (of the military, of children, etc.)

secret society

those (referring to things distant from the speaker)

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

sole of the foot

An underhanded trick. / A lifehack.

(literary, rare) here

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