The "Back Forty" was a term originally applied to RKO Pictures' film studio backlot in Culver City, California. Films such as King Kong (1933) and Gone With the Wind (1939) were filmed here. However, the phrase "I'm going to check out the back forty," usually implies that the speaker is going outside for a cigarette. The term, when in regards to smoking, is not a popular phrase among young people.
"Hi, mom! Good to see you! Why don't you make yourself a cup of coffee while I run outside?"
"You're going out for a cigarette, aren't you?!"
"Nah, I'm just going to check out the back forty."