An expression used to describe an activity typically done in secret, in a hidden place. Probably a socially unacceptable thing. Like the kinds of things children do with their friends, beyond the watchful eyes of their parents.

It was late in the morning, and the streets were empty as everyone else was off at work. My girlfriend and I had skipped breakfast in our haste as we set off for a road trip. Our hunger caught up with us along the way, so we stopped at a grocery store to buy some snacks.

After an older man in line requested a pack of cigarettes, his face contorted with shame. He looked at us, embarassed, and expressed his hope that we weren't smokers. He also apologized for setting a bad example for young people such as ourselves.

And then, seemingly out of context, he said, "In my day..." (Yes, just like the grumpy old man from Saturday Night Live.) "In my day, smoking was a behind the barn sort of thing."