"Necro" is a Greek word meaning "death", we use it as a prefix. For example necromancy. Originally necromancy meant divination, specifically by using the death (such as contacting ghosts) (things ending with "-mancy" refers to methods of divination, the prefix refers to the method used, like fire, the clouds, minerals, etc). Now necromancy refers to fictional magic which permits manipulation of the death, like reviving them, making them slaves, invoking death upon enemies.
Necroing, or necroposting, refers to the latter definition, where one poster "revives" a "death" thread by making a post weeks/months after the last one, thus bumpting the thread to the first page.