Imagine a situation where an employer offers an employee a contract that stipulates that all disputes should be resolved by some arbitration panel in a tax haven. I don't know about America but I am pretty sure that most/all Western European courts would ignore such a clause and apply the local labour laws in case of a dispute because it would be considered unreasonable that a strong part (the employer) could force a weak part (the employee) to enter a contract that favoured the strong part (it is usually very expensive and difficult to engage a arbitration panel in a foreign country).
Another similar situation would be that a big company, like Apple or Google, in their terms and conditions for a consumer service prescribed that all disputes should be resolved as in the previous example. Again, I think most courts would ignore such a clause and instead apply the local consumer laws for the same reasons as in the previous example.
A third example could be parents and their minor children. I have some difficulties coming up with a good example but say that a grandparent willed a large amount of money to a minor grand child and that in the current jurisdiction A, the parent's aren't allowed to access this money. The parents therefore decide to move to another country B that allows them free access to their children's money (and waste it while the child still is a minor).
Finally the child returns to A and seeks compensation. I reckon a court in A would apply A's laws to the situation and conclude that the move to B benefited the parents in an extremely one-sided and unfair way since the child had no means to object to the decision to move, and as a consequnce was robbed on her/his money.
If the situation had been a married couple (or soon-to-be married couple) where one party X had a lot of money that the other party Y had no access to in A but free access to in B and the couple voluntarily and consensually moved from A to B, I doubt that X could seek compensation from Y for spending the money because they are both adults with full legal rights and responsibilities and if they decide to marry and live in B rather than A, X can't afterwards return to A and demand that the courts in A should overrule the laws in B.
The third example(s) is quite artificial so no need to nitpick on details...
But back to the question. If I generalize the examples above there is one strong party and one weak party. The strong party, more or less, forces the weak party to enter an agreement that heavily benefits the strong party. Such a contract would in many cases not be upheld by a court because it is considered unfair. Is this "principle" called something?
I am interested in answers that go all the way back to the Roman legal system and up to current "Western" legal systems.