Doing a dad is an act that is hilarious to those all around you but their laughter only drives you into the downward spiral known as doing a dad. Doing a dad shoots your emotive side into overdrive warp speed. It can start off small, but the tears keep coming until all that is left of someone doing a dad is a blithering mound of tears sniffles and soiled handkerchiefs. All it takes is a few jokes at the person's expense like prodding their man-breasts and saying "Gor blimey, they're like titties them, tubbs", then the water works come on much to the amusement of the audience.
The phrase comes from a real person, with real emotional displays, who regulary did a dad. Do a dad is now a common phrase all over the world, usually preceeded by don't.
Chris: "Waddle waddle! Ha! Bill, you walk like a penguin!"
Bill: "MMnnn. Boo hoo! <cry cry cry>"
Tom: "Oh god, don't cry about it!"
Chris: "Oh! C'mon don't do a dad."