crab mouth is a popular expression used in the Portuguese language when someone commits to keep secret about a certain subject, closing your mouth and not telling anyone.
When an individual tells someone else something confidentially, they are usually asked to be careful to "keep crab mouth". This expression is also widely used when someone tells a gossip about a certain subject, immediately asking for "crab mouth", that is, to keep it a secret.
Examples: "He got a new job, but he's keeping his crab mouth" or "Everyone should keep their crab mouths about my mom's surprise party."
Origin of the expression "crab mouth"
This expression originates from the anatomy of the crab, a crustacean that lives predominantly in salt water. This animal has a very small mouth, which is hardly visible to the naked eye. Another feature of the crab's mouth is the small pincers that the crustacean uses to capture its prey, holding them with its mouth and hardly letting go, not even when it is killed.
Due to these two characteristics of the crab - having a small mouth and difficult to open - the expression was adopted by the culture popular, making a connection with the need to ask to "keep your mouth shut", just as the crab does when it catches a animal.
There are still other well-known popular expressions that are also used in the sense of "keep secrecy", like "mouth is a grave" or "shut up is medicine".
See also the meaning of "With the mouth in the bottle".