Cramsome bread

[Ananko-san]

I have been re-reading "The Adventures of tom Bombadil" and came upon the term cramsome bread. In my research I found out, that Tolkien coined the term cramsome. But I can't help wondering what kind of bread he envisioned. I feel like I am missing some kind of background knowledge a native english speaker would have. I have already tried consulting my german translation, but that was no help since it just germanises the phrase.
So I thought of asking everyone here:
What kind of bread do you think cramsome bread is?
Do you have any ideas what Tolkien based it upon?