Play it by ear

最近要还几本书给朋友，顺便约他吃个中饭，email 回来说不知有没有空，因为 "It's going to be a wild week." 建议 "Can you come by around lunch time and can we play it by ear?"
每次听到 play it by ear, 总是有点纳闷，又一直没去查看来源，你知道为什么说 play it by ear 吗？