Ecclesiastes 10:20 Do not curse the king, even in your thought; do not curse the rich, even in your bedroom; for a bird of the air may carry your voice, and a bird in flight may tell the matter.
The final admonition to his readership before chapter ten concludes is a warning not to curse the king or the rich, whether you are in private or even in your own thoughts. The idea of cursing here simply refers to speaking ill of them. If you are a disgruntled employee hating on your manager, in a sense this warning is befitting you.