The definition of an adjective is a part of speech that is used to describe a noun. Red, tall, better and fast are all examples of adjectives.
In modern parlance, a gentleman (from gentle + man, translating the Old French gentilz hom) is any man of good, courteous conduct. A gentleman may also simply be a polite term for any man, as in indications of gender-separated facilities, or as a sign of the speaker's own courtesy when addressing others. The modern female equivalent is lady.
Gentleman's clubs were private places that were designed where men can relax and create friendships with other men. In the 19th and 20th centuries, clubs were regarded as a central part of elite men's lives. They provided everything a regular home would have. Clubs were created and designed for a man's domestic needs.