The usual form of the metaphor is "a crab bucket". The mental image this is meant to evoke is that if one crab attempts to escape the bucket by climbing the si

The usual form of the metaphor is "a crab bucket". The mental image this is meant to evoke is that if one crab attempts to escape the bucket by climbing the side, then others will grab it, and their weight will pull it back down. So no crab escapes the bucket. A metaphor for unconstructive criticism or obstruction that squashes attempts at betterment.