The second old man threw his axe down pettishly, spat in the direction of the closed door and went off among the stacks of cordwood.
The door of the shack opened, the man in the Mackinaw poked his head out of it. "Sewer crabs is all," he said, and slammed the door again. I put my dollar in my pocket and went back up the hill. I figured it would take too long to learn their language.
[from 'Goldfish' by R. Chandler]
It is difficult to find out any specific meaning of 'sewer crabs' as used above in any kind of dictionaries.
My guess is:
The old man is just grumbling, and he is talking about something to eat in his shack. In fact he is complaining about eating same crabs everyday.
I heard the sewer crab is kind of fresh water crab, and sometimes live on in sewer.
But the location of the above dialogue is in Westport, facing the Pacific, where sea crabs are abundant. (a western-tip of USA) So no point in eating sewer crabs there. (I've never been there actually.)
My point is 'it is a cursing remark, not meaning any real sewer crabs'.
Hoping my long explanation just makes a bit of sense.