A hare-brained plan or idea is very silly, and not likely to work. People say it about schemes, not usually about a person as a whole: a hare-brained idea to get rich, a hare-brained scheme. It is informal, a little old-fashioned, still used when you want to be sharp but not cruel. You will also see it written harebrained, as one word. It comes from the old talk that a hare is a mad, jumpy animal — people still say mad as a March hare. Though actually, hares are not foolish; we just borrowed their bad name.

荒唐的、不切實際的（多形容計畫、點子）