From Tolkien Gateway
Dimholt, was a small fir-wood of dark trees within Dunharrow, outside the entrance to the Dark Door.
The Dimholt was traversed by a double row of standing stones that marked an ancient roadway leading away into the east, towards the Dwimorberg and the Paths of the Dead. Deep within a wood of black trees stood a single great stone to mark the way, and beyond that, among the roots of the Dwimorberg, lay the Dark Door of The Dead.
The word is supposed to be Rohanese and the element dim has the archaic sense "obscure, secret". Holt means "wood" in occasional poetic use (also seen in the name Everholt), with a cognate in German "Holz" meaning 'wood, lumber'.