This phrase is worth noting precisely because it does not belong in this dictionary: it makes sense in a moral universe that has utterly vanished. The last "cad and bounder" died, perhaps, about 1947 (see London Daily Telegraph obituaries for further evidence).
Although they are appropriately linked, the precise meanings differ. A "cad" is one who does harm to a woman's honor or sense of self-worth as, for example, by taking her for a garden walk when he has no intention of marrying her. A "bounder" is a presumptious upstart, seemingly ignorant of, but perhaps merely indifferent to, fundamental norms of propriety.
When your girlfriend's, or a friend who's a girl, or a female who walks by you, smells like someone opened a can of tuna coming from the vaginal area. It normally means she is in desperate need for a douche. Or a shower. And use lots of soap. Because that stench is hard to get rid of.
Two guys walk past a girl in Wal-Mart
Guy 1: DUDE! Did you smell that?!
Guy 2: Yeah bro. That was definitely a Flounder Bounder.
Guy 1: Hopefully she's making her way to the douching isle.
A person who has reached the pinnacle of whatever it is that can be, this person is beyond time space and existence as we know it. To be called this is confirmation that you are one of the most based individuals to ever exist.