What does « Mark / Stigma » mean in the Quran?

Az-zanim and al-muzannam: one who attaches himself to a group without belonging to it, like an appendage that grows without truly belonging.

The Almighty says: 'cruel, after all that, a usurper of lineage' (Al-Qalam 13), meaning known for evil as one recognizes a marked sheep.

It may also refer to one of doubtful origin who affiliates with those who are not his ancestors.