clandestine, secret or furtive
The backstairs (stairs at the rear of a building).
Confidential; private.
Having to do with concepts that are highly theoretical and without obvious practical application; often with mystical or religious connotations.
Intended for or likely to be understood by only a small number of people with a specialized knowledge or interest, or an enlightened inner circle.
One who believes, or is an initiate, in esoteric doctrines or rites.
An esoteric doctrine or treatise; esoteric philosophy.