Necrobumping means bumping a thread that has been inactive for more than a month. It is a discouraged practice on incels.co (sometimes even provoking a warning), partly because some old threads may be embarrassing, e.g. due to the OP's falling out of favor, or the rules having changed to make the thread rule-breaking.
For example, there was a thread on Incels.co, from very early in the site's history, asking if there were any incels who had children. One of the mods (probably blickpall) expressed the view that someone with kids could be incel. This thread got necrobumped much later, and was taken down because the culture of the site had changed so drastically that people were amazed such a thread could even have existed at one time in Incels.co's history.
Another example was, there was a thread where a famous user, whogivesafucc, who later got outed as a fakecel was ranting about fakecels. This got necrobumped much later and people noted that it had not aged well.
Necrobumping is a petty infraction but can easily serve as part of a ban rationale by mods who say, "Well, he has three dozen warnings in his profile so obviously has trouble following the rules."