A rogue, or bounder. A cad is a man who is aware of the codes of conduct which seperate a gentleman from a ruffian, but finds himself unable to quite live up to them. Cads are quite capable of disguising themselves as good chaps for some time, only revealing their true nature in circumstances of particular stress or temptation. Others embrace their caddishness whole-heartedly and delight in behaving in a manner which is, to be quite frank, not cricket.
They are certainly intelligent, educated, often cultured and frequently very witty, but, alas, are simply unreliable.
"Even by the standards of Permanent Secretaries, Appleby really was rather a cad."