button-down
1. adjective Having or fastened together by buttons, as of a shirt or shirt collar. Primarily heard in US, South Africa. We're going to a nice restaurant for dinner, so be sure to wear a button-down shirt, please!
2. adjective Conservative, unimaginatively conventional, or old-fashioned, as of a person's clothing, attitude, behavior, or beliefs. Primarily heard in US, South Africa. He spent his entire adult life as a button-down clerk, so when he turned 65, he decided it was time for adventure.
3. noun A shirt that has a collar and buttons extending down the full length of its front. Primarily heard in US, South Africa. I've got a date tonight, so I'm going to buy a nice button-down for the occasion.