It is often used in a scenario to highlight how serious you are about a matter that would otherwise be regarded as humorous. It can also be used in a sarcastic manner. For example, when you are reply to a girlfriends statement.
Most no jkokes are not prepared as jokes they are said in the moment and really should have not left the mouth.
so in conclusion a No joke is a joke that is not funny and its infact shameful.
"that was such a no joke!, lol yeah sorry was funny tho, still a huge no :)!"