Since the mithral versions of a chain shirt and a breastplate (see Mithral Armor in the DMG) specify they can be worn under normal clothes, I had initially presumed this to be a feature unique to these magic items. However, upon reading the description for a chain shirt:
Made of interlocking metal rings, a chain shirt is worn between layers
of clothing or leather. This armor offers modest protection to the
wearer's upper body and allows the sound of the rings rubbing against
one another to be muffled by outer layers.
It seems to suggest that a normal chain shirt can also be worn underneath other layers, but doesn't specify 'normal clothes' like the mithral chain shirt does. Can a mundane chain shirt be concealed under light clothes like a shirt?
Best Answer
IRL chainmail was typically worn over a type of Gambeson or padded armor. Which would make it very bulky. Whereas you could wear an oversized tunic or something over it, it would be very obvious you were wearing something more than just clothing.
Here are a couple of great clips from Shadiversity on YouTube on Gambeson and Chainmail.
A chain shirt is just the part of the chainmail that covers the torso and arms. Having woven chainmail myself even being very careful there will be burrs and such so wearing this over normal clothing would be a problem and part of the protection of chain comes from the gambeson worn beneath just like part of plate's protection comes from the chainmail beneath it.
These are, of course, real life examples. But this is a fantasy game and anything not covered in the rules and descriptions of the items is left to the DM, obviously (and even anything actually covered by the rules is left to the DM). Myself I take a bit of a cue from history and made adjustments to my own games to remove things that are way over the top, but left in things that I could deal with.