The decade of the 1840s, 1940s, etc.
The decade of one's life from age 40 through age 49.
They're both in their forties.
(temperature, rates) The range between 40 and 49.
What to wear when the weather is in the forties really depends on whether you're talking fahrenheit or celsius.
From or evoking the 41st through 50th years of a century (chiefly the 1940s).
Two forties aircraft sat forgotten in the hangar until they were rediscovered by two trespassing teens.
A bottle of beer containing forty fluid ounces.