Do you mean an App created with Odoo Studio?

If so, in Developer Mode, visit:

Settings --> Technical --> User Interface --> Menu Items

Find the name of your App (actually you are finding the Menu to your App) and then delete it and any child Menus.

Although this will still leave behind models and views, it will remove the App from the Apps Switcher dashboard:

Before:

After:

Hi,

You can delete the custom app from the under the apps menu by switching the tree view and select the app and action-> delete. 

If custom app is installed in database, delete the app only after uninstalling the module.

Then you can remove the module from the addons path.

Thanks