half-baked
PRONUNCIATION: hăf' bākt', häf'-
ADJECTIVE: 1. Only partly baked. 2. Informal Insufficiently thought out; ill-conceived: a half-baked scheme. 3. Informal Exhibiting a lack of good judgment or common sense: a half-baked visionary.
One of my all-time favorite quotes is from the movie The Graduate starring Dustin Hoffman as Benjamin Braddock:
Online Etymology Dictionary:Father: "Benjamin, is this another one of your half-baked ideas?"
Benjamin: "No, this one's completely baked."
So, half-baked does not seem as old as harebrained.half-baked in sense of "silly" is from 1855