The Mechanics and Rhetoric of Conversation : Page 43

On the other hand, to apply critical or unpleasant comparisons to types of persons who are guilty of things which can and should be corrected is not really slang but picturesque language. It should not be discouraged as long as it remains within the bounds of good taste. Calling a rough person a mugg, a spiteful girl a cat, a parasite a chiseler, an unsociable person a crab, an immoral woman a bag, ought not to be discouraged as slang. These are a picturesque way of disapproving and perhaps correcting social faults. Some eventually become standard for designating an evil thing, such as parasite in the list just mentioned. Originally meaning eating beside another, it is now a standard term for one who dishonorably lives off another's labors. Conversationally, these picturesque comparisons are desirable and necessary. One must remember, however, that the more picturesque a term is the more one must guard against overusing it; it calls too much attention to itself to allow frequent use.