Quite right, Turlough: it's a stone weight used as an anchor, sometimes held in a wooden framework, or simply tied on the end of a rope. I've always assumed that in Amphiteite's case, they would have used a proper anchor of wood and iron, and "killick" is used either ironically or informally.

BTW, the World Wide Words site is an excellent place ti look for things of that sort. There's a weekly newsletter too, if you like that kind of thing.

Steve