What does "뱃사람의 계집은 뱃사람다워야 한다" mean?

ANSWER: "The wife of a sailor must be like a sailor."

In the expression "뱃사람의 계집은 뱃사람다워야 한다," the word 뱃사람 means sailor, literally boat (배) person (사람); the word 계집 can mean either woman, wife, or mistress; the word -답다 attaches to nouns and means to be like; and the ending -어야 한다 means must.

The expression means a wife should adopt the ways of her husband. To help better explain the expression, here is a story:

A maiden (처녀) of a poor but noble family (“yangban” 양반)) married a sailor (뱃사람). The food, clothing, and customs of her husband’s life were naturally different from those of her yangban family (친정), but she became comfortable with the customs of sailors.

The labor pains (진통) started sometime before the sailor’s wife was ready to give birth (출산). During a difficult birth (난산) in a yangban’s house, if the wife grabs hold of her husband’s topknot (상투) and pulls hard, the baby slides out easily, but that is not the custom of sailors. Instead, sailors sit next to their wives and repeatedly sing part of a song sailors sing as they struggle together to pull in a fishing net: “어기여차, 어기여차, 어기여차,” which translates in English as "Heave-ho! Heave-ho! Heave-ho!

The story comes from the following book. The rough English translation is mine.