Proverbs are short sayings that encapsulate some generalized truth or advice that is easily understood. One is provided near the beginning of the story, as Quatermain is apologizing for his lack of professional writing ability, but believes the story itself will make up for any lack of finesse in its telling. He then relates the Kukuana proverb, "a sharp spear needs no polish." This is meant to imply a thing well-suited for its task, such as a good story, will do the job well regardless of any additional decorations.

Idioms are terms that tend to have meaning beyond their literal definitions, and often require some degree of education in their use; for example, "raining cats and dogs." One is mentioned in Quatermain's description of Captain Good, and his impressions of naval officers, which is generally positive, but being assured there must be some "black sheep" among them. This is an idiom for those that don't fit into the group, generally with a negative connotation that implies a lack of morals or conduct. Another is "salted" cattle, referring to the animals Quatermain purchases, meaning animals that have survived certain diseases and are therefore immune to them.