Yours, Etc.: Origins and Uses of 8 Sign-Offs
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 World War II application to resistance movements against German occupation, on analogy of the dominant culture and Nazis. Noun sense of "underground railway" is from 1887 (shortened from phrase underground railway, itself attested from 1834).