Is The trope that wizards can't use armor just an arbitrary limitation?

[The Colonel]

This sort of thing seems to assume a class based RPG - I've only encountered it outside class based games in a context where it appears to be a deliberate attempt to mimic the same effect.
The main reason seems to be either PvP balance or niche protection - explanations given as fluff rarely make any sense.
And of course, there's "that one specific class based RPG" with its origins in wargaming where "magic users" and "fighting men" were simply different kinds of unit with different capabilities, without needing to probe too far into the underlying reality of the thing.
The whole "you can't do it at all because stuff" position is distinctly old school - the modern trend (and the same trend that has always existed in non-class systems) would be towards "you can try, but you'll suck at it" ... which seems far more congruent.

Mythically speaking, wizards tend not to wander about in armour (although sword wielding seems reasonably common), but on the other hand they tend not to shoot fireballs either. Also, most mythic wizards are (or appear to be) very old, which may also impair their fighting ability.

[Michael Cule]

The 'no metal' thing appears in EMPIRE OF THE PETAL THRONE too and I'd give a pretty to know if the Professor stuck that in there before he found DnD or after.

It's mostly a niche-protection rules based thing rather than a mythical thing. GURPS magicians mostly want to avoid carrying heavy loads because that cuts into their fatigue points that they're going to be using to power spells. Which doesn't say 'you can't' but rather 'if you do, you're going to have live with these problems'.

[Polydamas]

Originally Posted by whswhs

Elric of Melniboné. Sheelba of the Seven Eyes and Ningauble of the Eyeless Face.

Bill Stoddard

And quite a few Robert E. Howard enemies and third parties. Dr. Karadur of Mulvan was too old and frail for armour, and Harold Shea didn't wear armour either ... but early L. Sprague de Camp seems to have got his fencing from the likes of Egerton Castle rather than Alfred Hutton.

[shawnhcorey]

Originally Posted by Irish Wolf

Can't speak to original D&D, but AD&D's first edition specified that metallic armors were incompatible with higher magics. They also claimed that mailed gauntlets would interfere with the subtle gestures needed to case spells.

Yes, that seems to be derived from folk tales the the magical fair folk, that is, the Fae, cannot abide iron in any form.

[smurf]

I thought it came from the fae myth that had weakness to iron.

[Irish Wolf]

Thinking back, the limitation on metallic armor in D&D does make at least a little sense - you don't want to go casting Chain Lightning while wearing what is in essence a body-hugging lightning rod...

[xerxes]

Originally Posted by whswhs

Elric of Melniboné. Sheelba of the Seven Eyes and Ningauble of the Eyeless Face.

Bill Stoddard

Ning has the seven eyes and Sheelba is eyeless...though I've made that same error myself when talking about Leiber's works.

[shawnhcorey]

Originally Posted by Irish Wolf

Thinking back, the limitation on metallic armor in D&D does make at least a little sense - you don't want to go casting Chain Lightning while wearing what is in essence a body-hugging lightning rod...

That would let the lightning go around your body, not through it. It should significantly reduce the damage you suffer.

[Disliker of the mary sue]

Hm interesting to hear this all so far, so in fiction wizards never really been armored but that just more or less not soldiers.

Yeah I just keep thinking the whole squishy wizard trope in rpgs is a one of those obvious rule patches that devolped so that classes other then magic users have a point to exist in a party. If magic user had the ability to have as high hp as a warrior and the ability to equip heavy armor without it really effecting magic then what would be the point of any class that did not use magic since at high level magic is very powerful.

[Murrkon5]

I'm 100% with Smurf's thought. "Cold iron" has been the bane of magic and fairie (however you want to spell it) action for a very long time. Yes, even before D&D!

In gaming terms, I've seen many a system (well, more than one) where spell slingers can wear armour, just not iron/steel. The rules then go on to list varieties of "silver alloy" plate for the well-dressed battle mage (with a deep wallet. That silver armour stuff ain't ever cheap!)