The term uses the Lating prefix 'pre', meaning 'before'. The 'app.' is the abbreviated 'application', and an 'ism' is, among other things 'a usage'.

We all say some funny things, frequently accidentally. People never let you forget these. A pre-app-ism is something which stands long after you've forgotten the reason. You wanted to fill something out, and you probably did, but the pre-app-ism lingers.