chimerical

1. Created by or as if by a wildly fanciful imagination; highly improbable.
2. Given to unrealistic fantasies; fanciful.
3. often chimeric Of, related to, or being a chimera.

adjective

1. Existing only in the imagination: chimeric, conceptual, fanciful, fantastic, fantastical, imaginary, notional, unreal, visionary. See real/imaginary.
2. Of, relating to, or in the nature of an illusion; lacking reality: chimeric, delusive, delusory, dreamlike, hallucinatory, illusive, illusory, phantasmagoric, phantasmal, phantasmic, visionary. See real/imaginary.

The adjective has 2 meanings:

Meaning #1: being or relating to or like a chimera
  Synonyms: chimeric, chimeral
  Pertains to noun: Chimera (meaning #1)

Meaning #2: produced by a wildly fanciful imagination