'Duty' in the Bible

Then Judah said to Onan, “Go in to your brother’s wife, and perform your duty as a brother-in-law to her, and raise up offspring for your brother.”

All the service of the sons of the Gershonites, in all their loads and in all their work, shall be performed at the command of Aaron and his sons; and you shall assign to them as a duty all their loads.

Now this is the duty of their loads, for all their service in the tent of meeting: the boards of the tabernacle and its bars and its pillars and its sockets,

“When a man takes a new wife, he shall not go out with the army nor be charged with any duty; he shall be free at home one year and shall give happiness to his wife whom he has taken.

“When brothers live together and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a strange man. Her husband’s brother shall go in to her and take her to himself as wife and perform the duty of a husband’s brother to her.

But if the man does not desire to take his brother’s wife, then his brother’s wife shall go up to the gate to the elders and say, ‘My husband’s brother refuses to establish a name for his brother in Israel; he is not willing to perform the duty of a husband’s brother to me.’

Then all the assembly replied with a loud voice, “That’s right! As you have said, so it is our duty to do.

The husband must fulfill his duty to his wife, and likewise also the wife to her husband.