Esoteric, abstruse; (in particular) taught orally to select students and not disseminated.
Based on lectures or exposition by monologue.
Cleverly contrived.
Tenuous; rarefied; of low density or thin consistency.
Barely noticeable, not obvious, indistinct.
Cunning, skillful.
Insidious.
Hard to grasp; not obvious or easily understood.