Proc Etymology?

"Metagaming is any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game. Another definition refers to the game universe outside of the game itself."

Meta short for Metagaming.

I absolutely use the term for chance effects like critical hits, the 30% chance of slow etc. from Midgardian Mail, and how things like crit apply to abilities like Made You Look, etc.

I don't recall a specific circumstance, but would say it's likely I've also misused the term, per the definition you stated.

I didn't know a "proc" was associated to a random effect.
According to this definition, I guess a critical hit could be considered as a proc.

I'd say that it's generally accepted that a proc is an additional effect that happens on hit.

And for your info, originally "meta" was an acronym. It stands for "Most Effective Tactic Available".
Nowadays, the meaning of meta is most likely referring to the word Metagaming.

In various MMO's it'd be more or less considered something like:

"A random chance of an effect triggering on something, generally a weapon or other piece of equipment, given certain stimuli."

That being said, the commonly described "chances on hit" (being a subgroup of procs) might sometimes have a 100% chance of happening given a certain circumstance, like dealing additional damage on critical strikes for example- and Polynomicon could be considered such case- having 100% chance to deal additional damage after using an ability.

Although I have yet to hear someone telling about "proccing" the Spear of the Magus; it'd be more of the case of "applying" its effect as it's just a debuff, and a stacking one on top of that.