You've never heard of Gandalf The Grey-Under-A-Yellow-T-Shirt?! Some "fan" you are...

In place of, which means he will be Donald Ducking it for the journey.

A wizard wears his shirt neither over or under his robes. He wears it precisely where he wants to. Which, in my case, is instead of underwear. Would you like to see?

Boromir responding to that: "It is a strange fate that we should suffer so much fear and doubt over so small a thing... such a little thing.

hit the inventory key and drag the shirt onto the chest slot..??

Definitely over