Cad, bounder, rotter mean one who shows himself to be no gentleman. Usually they are somewhat vague terms of contempt for bad behavior or manners.
Cad is applied especially to a man who violates in some way or another the code of morals or of manners by which he has been brought up and is supposed to be guided.
Bounder usually applies to a man who apes the gentleman but who in some definite way (as undue stylishness of dress or faulty habits of speech) marks himself as a mere imitation; the term condemns him as ignorant, obtrusive, or vulgar in a measure that puts him beyond the pale of good society.
Rotter may be applied to a man who is extremely objectionable especially on moral grounds.
The term is sometimes used of one who is felt as objectionable because of some failure, however great or small, to conform and then may suggest no more than mild disapprobation.