to stand up for someone
to support or defend a person when they are being attacked
to stand firm
to not change your actions or opinions
fight back
Defend yourself, resist an attack.
to stand up to someone
to not allow yourself to be treated badly, especially by someone in authority
fed up
bored or frustrated and unhappy (especially with a situation which has gone on too long)
give in
to admit defeat; to reluctantly stop arguing
back down
yield in an argument; withdraw your position