None of the potential substrings make any sense to me, so it’s one long jumble.
Wikipedia says "enclosure dam", which is more accurate since a dam has water on two sides, as the afsluit-dijk does, but that's not what we call it in Dutch. Then it would have been afsluitdam -- yes, dam and dam are the same in Dutch and English, just like dike and dijk are very similar. Wonder why! :-)
Edit: not sure why you're being downvoted?! I always find it rude when people downvote without comment (also on stackoverflow, I typically comment when I see it on my own or someone else's posts).
Edit: https://nl.wikipedia.org/wiki/Dijkstra (English page does not contain the important part)
Apparently -stra comes from some germanic word meaning "owner". I suppose Hoekstra owned the corner house and Dijkstra the dike.
-- https://nl.wikipedia.org/wiki/Dijkstra (through Google translate)