Mental inlining is an activity which is sometimes needed in order to understand code. You do it when you mentally take some piece of code (typically a method body) and put it somewhere else (typically a method invocation).
If some code is only understandable in the context where it is invoked, the activity which is necessary in order to understand this code is to virtually take it and put it at the place of its invocation. It is essentially the same operation as inlining a variable or a method butr without actually doing so.
Having to do mental inlining is a sign that there is a problem with the code. Either the variable or method carries a wrong, misleading or otherwise not understandable name. Or the whole concept the variable or method represents is wrong and should be revised.
Discuss this wiki article and the term on the corresponding talk page.