[un.der.ground] adv (1571) 1: beneath the surface of the earth
2: in or into hiding or secret operation
[2]underground n (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 esp. 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 esp. against a civil order c: an unofficial, unsanctioned, or illegal but informal movement or group; esp: a usu. avant-garde group or movement that functions outside the establishment [3]un.der.ground adj (1610) 1: being, growing, operating, or situated below the surface of the ground
2: conducted by secret means 3 a: existing outside the establishment "an ~ literary reputation" b: existing outside the purview of tax collectors or statisticians "the ~ economy"
4. a: produced or published outside the establishment esp. by the avant-garde "~ movies" "~ newspapers" b: of or relating to the avant-garde underground "an ~ moviemaker" "an ~ theater"