very thin, as thin as a hair
A fishing line made from hair.
The line along one's forehead where hair starts growing.
A very thin line in writing, drawing, or typography.
A very thin fissure.
Barely noticeable, not obvious, indistinct.
Tenuous; rarefied; of low density or thin consistency.
Cleverly contrived.
Cunning, skillful.
Insidious.
Hard to grasp; not obvious or easily understood.