Expression used to make fun of couples who are overly (and inapropriately) affectionate in public, and hopefully get them to stop.
Sometimes used to poke fun at friends who aren't together but always argue and tease each other in front of their other friends, by implying they act just like a couple.
"Look at Jim and Mary makeing out in public again. Get a room already!"
After listening to David and Sandy banter back and forth for 30 minutes. "Oh get a room you two!"