adj.
1.under the level of the ground
2.conducted with or marked by hidden aims or methods
3.used of independent armed resistance forces underground adv.
1.in or into hiding or secret operation
2.beneath the surface of the earth underground n.
1.a secret group organized to overthrow a government or occupation force
2.electric underground railway
Idiom of the Day
take (something) on the chin
to experience and endure a direct blow or assault
The man always takes things on the chin and he never complains.