It is jolly decent of me to help you. Will you have the courtesy to thank me? :) More simply, it is very good of me to help you. Will you be polite and thank me? :)

"Decent" also has a specialized meaning. "Indecency" can mean "nudity," and "decent" can mean "adequately clothed or covered." In the United States, under some situations, a natural question "are you decent?" It simply means "do you have enough clothing on?" Thus, one of the meanings of "decent" is "modestly covered, not nude." In situations like changing into a bathing suit near a beach, or changing into a costume for a theatrical play, you may have a situation where people may be dressing or undressing and doors may not be locked. It is fairly natural to yell "Are you decent? Can I come in?" In this case, "decent" is not a very strong word and doesn't carry much emotional weight. "Are you decent?" is informal language that would be used between people who are on a first-name basis. One might imagine a conversation: "Hey, I need to get the cooler. Can I come in? Are you decent?" "Just a second, I'm still changing. Let me grab a towel. OK, I have a towel around me, you can come in for a second." Laws regarding nudity often use the word "decent," and here it is very serious--but in the United States it does not correspond to non-legal use any more because we no longer feel as strongly about nudity as we did in the days when the legal language was formulated. If you read in the papers that somebody was arrested for the offense of "indecent exposure," they are in fairly serious trouble. This range of meanings apply only to the word "decent."

"Decency" is not old-fashioned, but it is somewhat unusual to find a situation where it is the appropriate word. "Courtesy" just means ordinary politeness. It can be fairly mechanical or insincere. It can be as basic as saying "Please" before a request. "Decency" is usually used in serious situation. For example: "I just read that airlines are taking advantage of people trying to leave Florida during the hurricane and that one airline was charging as much as $2,370 for seats." "What's wrong with them? Don't they have any sense of decency?" The airline was not just being impolite, it were doing something that was really wrong and unjust. If somebody walked up to a ticket counter, and the agent said "Good morning, sir. How are you? How glad to see you are in our Flying Fig Program. Yes, we would be delighted to put you on a flight out of Florida this morning, we have one available at 11:10 a.m. and the fare will be $2,370. We take all major credit cards and you will be credited with 10,000 frequent flyer miles. And you can wait in the Flying Fig Lounge where we will give you a free Pepsi." They would be acting courteously, but without decency.

Decency and courtesy are synonymous with one another. Is it decent of someone to hold the door for you, or are they being courteous? When in reality it's both. To use references from other answers here: If women are sunbathing in Europe, it is a completely acceptable and decent act because it's legal. However, it would be courteous and decent if they were left alone and not harassed or ogled by other civilians. In the end, both words are built on the morality of the individual using them. So if the person using these words thinks that they are royalty, they may have higher than normal standards for everyone around them and if, for example, they are buying something and the cashier doesn't give them the best service they've ever had, they might think that they were intentionally being indecent or uncourteous. While someone with more modesty, who doesn't expect anything special from anyone may find decency/courtesy in someone who simply holds a door open for them or is simply nice. If you're looking for a clear separation between words defining "right and wrong", you should learn the difference between ethics and morals. Ethics are based on a society's rules (i.e. business ethics, police ethics, political ethics, etc.), while morals are built solely on the individual and may change depending on experiences they have.

I think (not sure) that maybe decency is dependent on morals, whereas courtesy is determined by ethics.