to post in a thread that has been "dead", by being inactive for a relatively long period.

In the US, slavery was officially abolished with the 1863 Emancipation Proclamation. No one bothered to tell all the slaves in Texas until June 19, 1865, a day commonly celebrated as "Juneteenth."

To bring life to a dead thread; to post in a thread that hasn't had any new posts for a considerable amount of time

In the context of the forums, the rootword of "necropost," which is "post," is most accurately defined as "a message published on a website." The prefix of the word, "necro-" most nearly means "dead." As such, the appropriate definition for a "necropost" (which can function as both a noun and a verb) would be "(v) the act of publishing a message on a website in a dead topic" or "(n) a message published on a website in a dead topic."

To post on a thread that hasn't been active or relevant in a while ("dead").

Having sex with mail sent to you by somebody that is dead by the time you receive the mail.