underground (adv.)

1570s, "below the surface," from under + ground (n.). As an adjective, attested from c. 1600; figurative sense of "hidden, secret" is attested from 1630s; adjectival meaning "subculture" is from 1953, from adjectival use in reference to World War II resistance movements against German occupation, on analogy of the dominant culture and the Nazis. Noun sense of "underground railway" is from 1887 (shortened from phrase underground railway, itself attested from 1834).

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Definitions of underground from WordNet
underground (n.)
a secret group organized to overthrow a government or occupation force;
Synonyms: resistance
underground (n.)
an electric railway operating below the surface of the ground (usually in a city);
in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'
Synonyms: metro / tube / subway system / subway
underground (adv.)
in or into hiding or secret operation;
the organization was driven underground
underground (adv.)
beneath the surface of the earth;
water flowing underground
underground (adj.)
conducted with or marked by hidden aims or methods;
underground resistance
Synonyms: clandestine / cloak-and-dagger / hole-and-corner / hugger-mugger / hush-hush / secret / surreptitious / undercover
underground (adj.)
under the level of the ground;
underground caverns
Synonyms: belowground