Following the trend of prefixing a genre name with J- to signify Japanese origin (eg J-pop, J-rock etc.), J-core is an abbreviated term for Japanese hardcore dance music.
Though many only associate this term with artists such as DJ Sharpnel who are well known for their very Japanese-flavoured anime/film sampling hardcore, the umbrella term can be used to describe many kinds of Japanese-flavoured hardcore, such as happy hardcore, freeform, speedcore and terrorcore.
This term can sometimes be used to describe hardcore punk music from Japan as well as just hardcore electronic/dance music.
Notable J-core artists include DJ Shimamura, Buzzmasta, M-Project, DJ Chucky, DJ Technorch, DJ Sharpnel and REDALiCE.
When you go to the bathroom during an instant messaging conversation and return without having to explain a delay in replying. Only applicable if the time you were in the bathroom is unnoticeable and you don't have to lie about your disappearance when you return.
Some people are easier than others to do this with; i.e. if a person is capable of talking for half an hour wanting a reply to make sure you didn't go on holiday, they are often easy targets for a stealth piss.
Person 1: "So how are you?"
*** person 1 walks off to go to the toilet without informing person 2 ***
Person 2: "I'm fantastic!"
Person 2: "I got paid earlier so I bought some really awesome shoes in town"
Person 2: "And then I bought like 5 new CDs"
Person 2: "Which I'm listening to at the moment"
Person 2: "Oh and I saw your friend from school"
*** person 1 returns from urinating ***
Person 2: "He said you should come out sometime"
Person 1: "Yeah I think I should sometime!"
(person 1 has performed a stealth piss, unbeknownst to person 2)