Some background, in case you don't already know it. From a Google search, I found that the lyrics are from a song entitled "The Seven Deadly Virtues," from the 1960 musical "Camelot," by Alan Jay Lerner and Frederick Loewe. The musical is based on the legend of King Arthur. "Camelot" was the name of King Arthur's Court. It is where "the Knights of the Round Table" met. The knights went on a quest in search of the Holy Grail. This legend is famous in the English-speaking world. It is so famous that anyone interested in English literature and culture probably should know something about it. The Wikipedia article, https://en.wikipedia.org/wiki/King_Arthur is a good starting point. The legend has inspired many works of art and literature. The first telling of it was by Sir Thomas Malory in 1485. It's in English despite the French title, "Le Morte d'Arthur," but the English is too old for modern readers to understand. It is difficult even when the English has been modernized! Mark Twain had fun with it in 1889, in "A Connecticut Yankee in King Arthur's Court." When I was a kid, I loved T. H. White's fantasy, "The Once and Future King." More recently, it inspired Guy Gavriel Kay's 1980s fantasy trilogy, "The Fionavar Tapestry." Monty Python made fun of it in "Monty Python and the Holy Grail." "Camelot" itself has resonances in the United States. It was popular in the United States when John F. Kennedy was president, and he loved the musical. The musical ends with a heartbreaking song. King Arthur, broken by tragedy, wishes: Don't let it be forgot, That once there was a spot, For one brief, shining moment That was known as Camelot. After Kennedy was assassinated, his widow referred to the song, and said that there would be other great Presidents but that there would never be another Camelot. Since then, the "brief, shining moment" of the Kennedy presidency has been referred to as "Camelot."

Song lyrics are poetry. Often they don't make sense. It is common for a lyricist to distort the language to fit the meter and the rhyme. (The meter is the rhythmic pattern. In a song, the words have to fit the music). Writing a song lyric is a little like working a crossword puzzle. A perfect lyric should sound like ordinary, natural speech, that just magically and effortlessly happens to rhyme. Yes, here he is talking about "courage" as if it were "a sport" like football or basketball. No, that doesn't quite make sense. The idea, though, is that he doesn't take virtue seriously. He is acting as if choosing good or evil were just a game. His whole attitude is ironic. Obviously the lyricist chose "sport" to rhyme with "rigor mort." But "sport" doesn't make much sense here. "Rigor mort" is a nonstandard shortening of the medical phrase "rigor mortis," the stiffening of the body after death. This also shows an ironic sense of humor. "Rigor mortis" is a frightening and depressing phrase. He actually is giving it a joking nickname. "Purity" here means obeying society's rules about sex and romance. A "yen" can mean a wish, a desire, or a yearning. "I have a yen for a pizza right now." However, "yen" sometimes suggests romantic desire. Therefore, the idea of purity itself being "a yen" is ironic. It means a desire not to have romances. He goes on to say that purity is "very restful now and then." So we know that the speaker normally has many romantic encounters. He has so many they are tiring and exhausting. He is impure, he does not really want to be pure, but now and then he needs a rest. By the way, "the seven deadly virtues" is a nasty joke, too. In medieval Western tradition there are "seven deadly sins:" pride, greed, lust, envy, gluttony, anger, and sloth (laziness). The virtues, the good things, are not called "deadly." So the speaker--who, I find, is the villain, Mordred--is turning everything upside down: good is bad, bad is good.

Looks like the writer is saying the virtues are a fool's game, not for him, doomed to failure. So... Sport can mean more than a game like basketball, it can mean any sort of fun, or a gamble. In this case, he seems to be saying that when you act with courage, you're taking a chance of getting killed, hence "rigor mortis". Noble yen can mean a noble desire, as yen is hunger, desire. So purity is a noble desire. When the author says it's restful now and then, they might mean that a person is cynically calling it "purity" when they take a short break from not being pure. In other words, taking a night off from drinking and lewd intercourse.