How is this expression used? I know it literally means "miracle in feathers", but what does it mean to call someone that? My girlfriend keeps calling me this, and she gave some basic explanation that I don't seem to have understood well enough. Thanks.
The expression is popular in their language (I mean girls and women). I never take the expression as negative or offensive. Often it means that your girl appreciates your action, but she isn't completely happy because you looked (maybe) funny and awkwardly. In general, the expression can have rather tender sense...
In fact, "Чудо в перьях" is the heroine of a folk tale. I have always thought it is Russian, but when I tried to find it on the Net now, the only version I found was Mordovian. Here it is (in Russian): http://www.tale-store.ru/skazki-narodov-mira/55-mordovskie-narodnye-skazki/749-chudo-v-peryah If it is too hard for you to read it, I can give the gist: a stepmother mistreats her stepdaughter and to make fun of her, forces the poor girl to wear a dress made of feathers. The girl flees from home, taking the best dresses of her stepmother, and meets the crown prince in the forest. The prince takes her to the palace to show her as a freak to his guests, but later sees her in nice clothes, falls in love with her and marries the girl. As you see, it is a little like Cap o'Rushes - at least, the bit about the girl's clothes. It can also be regarded as a variation on the Cinderella theme.
So, in this case, "чудо" would be better translated not as "miracle", but as "curiosity", "oddity", "absurdity", "strange thing" or something like that. The poor girl in her dress of feathers looked not miraculous, but preposterous.
Thus, if anyone calls you "чудо в перьях", he means that either your appearance or behaviour (or both) is absurd.