Not such a stupid question at all! First, I'm assuming you're talking about WAR files here, because for EAR files the context root is obviously part of your EAR - specified in application.xml.
For WARs, though, you will indeed not find a "contextRoot" property on a generic jee.War, because whether the context root can be specified or overridden depends on the target platform.
If you're deploying to WebSphere you can declare your WAR file in your deployment package as a was.War rather than a jee.War. You can then set its contextRoot property  as follows:
Hope this helps!