megacity (n.)

before vowels meg-, word-forming element often meaning "large, great," but in physics a precise measurement to denote the unit taken a million times (megaton, megawatt, etc.), from Greek megas "great, large, vast, big, high, tall; mighty, important" (fem. megale), from PIE root *meg- "great." Mega began to be used alone as an adjective by 1982.

High-speed computer stores 2.5 megabits [headline in "Electronics" magazine, Oct. 1, 1957]

c. 1200, from Old French cite "town, city" (10c., Modern French cité), from earlier citet, from Latin civitatem (nominative civitas; in Late Latin sometimes citatem) originally "citizenship, condition or rights of a citizen, membership in the community," later "community of citizens, state, commonwealth" (used, for instance of the Gaulish tribes), from civis "townsman," from PIE root *kei- (1) "to lie," also forming words for "bed, couch," and with a secondary sense of "beloved, dear."

Now "a large and important town," but originally in early Middle English a walled town, a capital or cathedral town. Distinction from town is early 14c. OED calls it "Not a native designation, but app[arently] at first a somewhat grandiose title, used instead of the OE. burh"(see borough).

Between Latin and English the sense was transferred from the inhabitants to the place. The Latin word for "city" was urbs, but a resident was civis. Civitas seems to have replaced urbs as Rome (the ultimate urbs) lost its prestige. Loss of Latin -v- is regular in French in some situations (compare alleger from alleviare; neige from nivea; jeune from juvenis. A different sound evolution from the Latin word yielded Italian citta, Catalan ciutat, Spanish ciudad, Portuguese cidade.

London is the city from 1550s. As an adjective, "pertaining to a city, urban," from c. 1300. City hall "chief municipal offices" is first recorded 1670s; to fight city hall is 1913, American English. City slicker "a smart and plausible rogue, of a kind usu. found in cities" [OED] is first recorded 1916 (see slick (adj.)). City limits is from 1825.

The newspaper city-editor, who superintends the collection and publication of local news, is from 1834, American English; hence city desk attested from 1878. Inner city first attested 1968.