|a scrap or morsel of food left at a meal.|
|a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison.|
|a fanciful or impossible plan or hope|
|[alluding to dreams produced by smoking an opium pipe]|
A fantastic notion or vain hope, as in I'd love to have one home in the mountains and another at the seashore, but that's just a pipe dream. Alluding to the fantasies induced by smoking an opium pipe, this term has been used more loosely since the late 1800s.