A vagina on a chubbier chick where the curtain portion is covered by the large outside. Instead the vagina bulges out a little and all you see is a line, resembling a worm on a rock.
A worm sucking doo doo head is just an example of a poor insult often used by preachers to show that insults hurt just as badly as being hit by a baseball bat. It's a nonoffensive insult.