I. ˌən-dər-ˈgrau̇nd adverb
Date: 14th century
1. : beneath the surface of the earth
2. : in or into hiding or secret operation
II. ˈən-dər-ˌgrau̇nd noun
Date: 1594
1. : a subterranean space or channel
2. : an underground city railway system
3.
a. : a movement or group organized in strict secrecy among citizens especially in an occupied country for maintaining communications, popular solidarity, and concerted resistive action pending liberation
b. : a clandestine conspiratorial organization set up for revolutionary or other disruptive purposes especially against a civil order
c. : an unofficial, unsanctioned, or illegal but informal movement or group ; especially : a usually avant-garde group or movement that functions outside the establishment
III. ˈən-dər-ˌgrau̇nd adjective
Date: 1601
1. : being, growing, operating, or situated below the surface of the ground
2. : conducted by secret means
3.
a. : existing outside the establishment
an underground literary reputation
b. : existing outside the purview of tax collectors or statisticians
the underground economy
4.
a. : produced or published outside the establishment especially by the avant-garde
underground movies
underground newspapers
b. : of or relating to the avant-garde underground
an underground moviemaker
an underground theater