An ass app is terminology commonly used by graphic designers when having to urgently create something from scratch or figuratively 'pull it out of your ass.'
Client: "Oh, we love the ad campaign but would like to make a few changes. Could you change the photo, rewrite the copy and have it to the publisher in an hour?"
Graphic Designer: "Sure! No problem. Let me use my ass app and I'll have that ready in a jiffy!"