Whistle down the wind
Send away or abandon.
The phrase 'whistle down the wind' is best known as the title of the 1961 film, directed by Bryan Forbes, and most people probably assume that it originated with the film. The plot revolved around the mistaken belief of a group of schoolchildren that a fugitive criminal they had discovered in hiding was in fact Jesus. In a tale heavy in Christian symbolism, the criminal was eventually inadvertently given away by the children and re-arrested.
The phrase is in fact much older and derives from the earlier 'whistle away', which meant 'dismiss or cast off'. This usage dates from at least the 16th century and was used, for example, in Nicholas Harpsfield's A treatise on the pretended divorce between Henry VIII and Catharine of Aragon, circa 1555:
Fourthly, whereas his authors write that St. Dunstane, Archbishop of Canterbury, ex communicated Earle Edwyn for marrying cognatam, that is to say his kinswoman, he hath translated it, for marrying his brother's wife. Fifthly, he belyeth our noble learned countryman John Bacon, saying that he was clapped and whistled out at Rome for maintaining this marriage
The 'down the wind' part of the phrase comes from the sport of falconry. When hawks are released to hunt they are sent upwind and when turned loose for recreation they are sent downwind. Thus, to 'whistle someone/thing down the wind' is to cast it off to its own fate. Shakespeare alluded to this in Othello, 1604:
If I do prove her haggard, Though that her jesses [leather straps] were my dear heartstrings, I'ld whistle her off and let her down the wind, To pray at fortune.
The first appearance of the phrase as we now know it that I can find in print comes from Blackwood's Edinburgh Magazine, 1826:
Surely someone who can whistle down the wind this painful weakness of his nature ... is an anomaly, not a man.