The origin of this phrase is believed to be from Aesop, a Greek fabulist who is said to have lived from 620 to 560 BCE. He wrote a number of different fables known collectively as Aesop's Fables. ( source ) Just like in the fable, The Boy Who Cried Wolf by Aesop, the meaning of the phrase means someone who lies but when they choose to tell the truth nobody will believe them due to their previous lies. 

According to the Oxford English Dictionary, the saying originated in a 17th-century British proverb about "weeping over shed milk." By 1738, the language had been modernized, with satirist Jonathan Swift writing, "'Tis folly to cry for spilt milk," in his treatise Polite Conversation.

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