Similar to the teeth lines, the fin ones reveal Mac's similarity to the killer shark, but with a strong focus on his appearance. According to the song, the shark gets red and bloody when he kills, but Mac looks spotless in his fancy get-up, giving the appearance of a safe gentleman rather than a dangerous criminal.
To combat the increasing callousness of mankind, J. Peachum, a man of business, has opened a shop where the poorest of the poor can acquire an exterior that will touch the hardest of hearts. (1.1.1-4)
Most shops help you improve your appearance, but Peachum's will sell you all the equipment to look as miserable, sick, and as poor as possible. Why, you ask? The better to beg, you see. By giving off the appearance of poverty, the beggars inspire more pity and make more moolah.
PEACHUM. The sight of such types puts a man into an unnatural state where he is willing to part with money. Outfit A: Victim of vehicular progress. The merry paraplegic, always cheerful—He acts it out.—always carefree, emphasized by arm-stump. Outfit B: Victim of the Higher Strategy. Outfit B: Victim of the Higher Strategy. The Tiresome Trembler, molests passers-by, operates by inspiring nausea—He acts it out.—attenuated by medals. (1.1.117-123)
Peachum names his outfits with euphemisms. Outfit A gives the appearance of being a paraplegic, by creating fake amputations. Outfit B puts medals on the beggar's chest to give the appearance of being a veteran. The euphemisms, "victim of vehicular progress" (a.k.a., has been run over) and "victim of the higher strategy" (a.k.a., a pawn in the nation's military strategy) are ironically positive spins on terrible circumstances.
PEACHUM. I see. White gloves and a cane with an ivory handle and spats and patent-leather shoes and a charismatic personality and a scar… (1.1.200-202)
All of the descriptors Peachum throws out to describe Mac are like flashing neon signs that say, "This guy's fancy!" White gloves mean that he doesn't work or get dirty, and an ivory-handled cane indicates a healthy cha-ching factor. The last item on the list, though, the scar, is a hint that fancy appearances can hide danger.
MAC. Eating his fish with a knife! Anybody who does that is just a plain swine, do you get me, Jake? Think about it. (1.2.233-234)
These guys have stolen all the furniture in the house, and Mac gets onto them for, wait for it, their table manners. He compares his friend to a pig because he doesn't know how to eat fish properly. Ever heard of honor among thieves? Well, this is a sort of Emily Post version of it, where keeping up appearances at the table is important, no matter what kind of crimes you commit elsewhere.
MRS. PEACHUM. Married? First you rig her fore and aft in dresses and hats and gloves and parasols, and when she's cost as much as a sailing ship, she throws herself in the garbage like a rotten pickle. Are you really married? (1.3.7-10)
Mrs. Peachum compares her daughter, Polly, to a ship: "rig her fore and aft" means to get a ship ready with all the ropes it needs, back to front. Polly's fancy dresses and accessories were like a ship's rigging. They were supposed to keep her sailing, even in a storm. But she's gone and gotten married to the wrong guy, which takes her from expensive ship to rotten pickle in no time.
PEACHUM. Of course natural scabies is never as good as the artificial kind. (1.3.117-118)
Scabies (can we just say, yuck?) is a contagious skin infection caught from tiny mites, and leaves quite an ugly rash in its wake. Peachum, who sells costumes to make beggars look more pitiful than they already are, would rather his clients use a phony rash. The appearance of a rash might inspire more charity than actual suffering.
MAC. In all other respects you will carry on exactly the same as before. Get up at seven, wash, have your weekly bath and so on. (2.4.54-56)
When he finds out that the law is after him, Mac knows he has to pack up and leave. But he instructs his new wife, Polly, to go about her business as though nothing has changed. This type of keeping up appearances is meant to cover up the crisis that the couple find themselves in.
LUCY. Oh, Mac, I only want to become an honest woman.
MAC. If you think marriage with me will…all right. What can a man of honour say more? He can say nothing more. (2.6.102-104)
"Making an honest woman" out of someone used to mean marrying her. Nowadays we hope that ladies make honest women out of themselves, but Lucy was a woman of her time and needed Mac to help her find her way in society. Of course, the position that marriage would give her is questionable when you think about the kind of guy she's hitching her wagon to.
POLLY. Yes, my dear, naturally the wife has…
LUCY. The wife
POLLY. …the wife is entitled to some preference. Or at least the appearance of it, my dear. (2.6.131-134)
Similarly to the previous quote, Polly recognizes that being someone's wife gives you certain privileges in society, and that being someone's girlfriend usually gets you a bad reputation and not much more. But our dear Polly is also smart enough to recognize that Mac isn't going to give anyone an honorable title. She's willing to put up with his running around as long as she gets to "appear" to take preference.