I was there when mmo players started to use the term "hardcore". Time for a Cool Story Bro!
It was 1998 and I was heavily playing Everquest. For those of you unfamiliar with Everquest raiding, it was a ton of players crawling their way through an exceptionally long dungeon to get at a very hard boss that had a low chance to drop an item. The average timeframe on most of these things measured in several hours...and that was if you didn't wipe.
Some players, like myself, were willing to spend the hours upon hours needed to drag a group into the depths of these hell holes. I cant even tell you how many raids I led to get Clerics their Epic Weapon. The players that were willing and able to do this, on weekdays, and sometimes 2 -3 times in a row were not common. The term "hardcore raiders" started to make the rounds. I can actually remember one of the first times someone said it to me, it was a descriptive value. "You've run this twice today already?! that's some Hardcore stuff right there".
Eventually the term was applied to anyone who was running the really high level raids that required massive time investment.
That's what the term Hardcore describes. Time investment per day. A casual player might hop on and play an mmo for an hour, maybe an hour and a half (length of an average movie). A hardcore player is going for 3, 4, 5 hours on a weekday and possibly doesn't sleep on a weekend preferring an excess of redbull and caffeine to pick up the slack.
Hardcore DOES NOT indicate anything to do with Skill or Experience. It does not make you superior to anyone or anything, it only describes how much time you spend playing the game in a single sitting.
So if you log in, run the hardest content in the entire game, and then log out until tomorrow when you plan to do it again? You my friend are actually CASUAL. The guy who runs all the roulettes + does a little RP + some crafting for shiggles and runs some dungeons on an alt? That's the HARDCORE one.