Chimerical
Etymology
From the Chimera, a monster of Greek myth stitched together from a lion, a goat, and a serpent, breathing fire — a creature so unnatural its name came to mean any impossible hybrid or figment. Because such a beast could exist only in imagination, "chimerical" came to describe fancies and projects with no grounding in reality.
adjective · /ky-MEHR-ih-kul/
Wildly fanciful and unreal; based on an impossible or imaginary conception.
What it names
A dream or scheme that could never actually exist — use it for hopes and plans that are not just unlikely but built on pure fantasy.
Example
His plan to fund the whole venture on goodwill alone was chimerical from the start.
Related idea
Utopian describes an ideal that is desirable but impractical; chimerical stresses that the thing is imaginary and impossible, not merely hard to achieve.