The origin of this phrase: The phrase "放鸽子(fàng gēzi)" mainly means that one doesn't keep his promise. The meaning of the word "放(fàng)" means sending something. And "鸽子(gēzi)" refers to a dove, a kind of bird. The phrase is from a story. In ancient times, people mailed letters by doves. One time, two people agreed to write letters to each other, but one of them sent a dove without letters. Then another people said: "why did you just send the dove? And you haven't kept your promise." So the phrase is used with the meaning that one hasn't kept his/her promises.