'1940s' originates from the Arabic numeral system, specifically the number '1940' with a pluralizing '-s' added to denote the decade.

'1940s' was commonly written earlier as '1940's' with an apostrophe; over time style guides favored dropping the apostrophe for decade plurals, resulting in the modern form '1940s'.

Initially it denoted strictly the literal years 1940–1949; over time it also came to refer to the cultural, stylistic, or historical characteristics associated with that decade.