from Wiktionary, Creative Commons Attribution/Share-Alike License
- I received the information from a source not to be overtly exposed.
The phrase has a somewhat contentious origin: some attribute it to Ecclesiastes 10:20 "Do not revile the king even in your thoughts, or curse the rich in your bedroom, because a bird in the sky may carry your words, and a bird on the wing may report what you say." Another explanation is a simple allusion to carrier pigeons or other such messenger birds. (Wiktionary)