The term the apostle uses here appears frequently in the Old and New Testament alike to describe the action of placing (oneself) under, that is, submitting. The meaning of the phrase is not in the first instance to obey (though that may be the consequence of submission) but is to surrender one’s rights, liberties, and preferences. This instruction is striking in the present context because household servants were vulnerable to the whims and wants of their masters.
18 Servants, be subject to your masters with all respect, not only to the good and gentle but also to the unjust.