It means Sasuke. Sasuke in that emo guy in Naruto, the anime.
So well, why sask1 you're asking? Huh, just read it! But be aware that this only works for french people since the '1' changes. So, it's like... - SASS-KK-UN -

By the way, one in french is " un ". So it makes sense , don't you think?

The following example will help you understand.

A buzz word being used to refer to creators and influencers forming or encouraging unhealthy relationships with their fans. Usually used in the context of young audiences feeling they have a relationship with the influencer due to the huge amount of the influencer’s content they consume. The influencer in question, however, is unaware of the fan’s existence other than the general knowledge of having fans. Common examples include Dream and his stans. These relationships are exacerbated by the influencers confirming they have more than a creator-to-viewer relationship.

The word is circulating due to various YouTube essays about the phenomenon using the word, and therefore people who enjoy complaining about influencers repeating what they heard in the video in any comments section they can find.