"i" is a meaningful variable name

No, really. You should always use meaningful variable names. But you should also remember that i is a variable name which means "I am an integer index". So you can -- and should -- use it where appropriate: i.e. for loops in a small scope.

Using a_idx or iIndex instead is just silly. Oh, and j, k, x, y and z are also meaningful -- when they are meaningful

You still have to explicitly declare variables that start with i-n (or any other letter) as an integer before using it in languages like C/C++. That's explicit type casting and C/C++ enforces strong type casting, so it's good coding practice.