Thicket
Etymology
From Old English "thiccet," from "thicce" ("thick, dense"). It is simply "thickness" made into a place.
noun · /THIK-it/
A dense growth of shrubs, small trees, or undergrowth.
What it names
An impenetrable tangle of vegetation — and serves as a ready metaphor for anything dense and hard to get through.
Example
The rabbit vanished into a thicket of brambles.
Related idea
A thicket is a tangle at ground level; the understory is the whole layer of growth beneath the forest canopy.
See also