Sindarin: forest-dark. A forest or wood in Minhiriath, at the mouth of the Baranduin.
The ErynVorn is probably a remnant of the vast forests that covered much of the west of Middle-earth in the First and early Second Ages. As Unifinished Tales recounts, much of these forests were cut down by Numenoreans for lumber. The invaders were resisted by indigenous folk (akin to the Dunlendings), but these were eventually driven out of Minhiriath or into the dark woods of ErynVorn.
A remnant of this "secretive hunter-folk" remained in ErynVorn until well into the ThirdAge, and possibly beyond it: the ErynVorn was by no means a small forest, being much bigger than Druadan Forest and roughly equal in size to Lorien, and more than capable of accommodating a population of Men. The Great Plague devastated Cardolan in 1636, but the Men of ErynVorn, being cut off from the rest of the world, would not have been affected. They would also most likely have escaped the worst effects of the flooding and harsh winters of the later Third Age.
If there were people in ErynVorn at the end of the Third Age, they would probably have enjoyed a lifestyle similar to that of the Woses of Druadan; physically, they would have resembled Dunlendings or Men of Bree.