Underground

wiktionary

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From Middle English undergrounde(adverb), equivalent to under +‎ ground. Compare Dutch ondergrond, ondergronds, German Untergrund, Danish undergrunds.

etymonline

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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).