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.
Of a person: having obtained worldly experience, and lacking naiveté; cosmopolitan, worldly-wise.
Complicated, especially of complex technology.
Of art or other things: appealing to the tastes of an intellectual or sophisticated (sense 1) person; cerebral; also, cultured, elegant, refined.