Meaning

noun a secret group organized to overthrow a government or occupation force

noun an electric railway operating below the surface of the ground (usually in a city)

adjective under the level of the ground

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

Etymology from Wiktionary, CC BY-SA 3.0