A quin is one of five babies born together, from the same mother, at the same time. The full word is quintuplet; quin is the short, informal British form. American English more often says quint. You almost always hear it in the plural — the quins — because the story is about all five, though each child is a quin. It is a newspaper and family word, not a hospital form: a doctor would write quintuplet. It has nothing to do with the family name Quinn, which only sounds the same.