1 - [adj] under the level of the ground;
2 - [ n] a secret group organized to overthrow a government or occupation force;
3 - [adv] in or into hiding or secret operation;
4 - [adj] used of independent armed resistance forces;
5 - [adj] conducted with or marked by hidden aims or methods...
Synonyms: belowground, clandestine, cloak-and-dagger, covert, guerilla(a), guerrilla(a), hole-and-corner(a), hugger-mugger, hush-hush, irregular, metro, on the quiet(p), partisan, partizan, resistance, secret, subsurface, subway, surreptitious, tube, undercover