UNDERGROUND

I. adverb Date: 14th century beneath the surface of the earth, in or into hiding or secret operation, II. noun Date: 1594 a subterranean space or channel, an ~ city railway system, 3. 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, a clandestine conspiratorial organization set up for revolutionary or other disruptive purposes especially against a civil order, an unofficial, unsanctioned, or illegal but informal movement or group, III. adjective Date: 1601 being, growing, operating, or situated below the surface of the ground, conducted by secret means, 3. existing outside the establishment , existing outside the purview of tax collectors or statisticians , 4. produced or published outside the establishment especially by the avant-garde , of or relating to the avant-garde ~