1.
If you fight back against someone or something that is attacking or harming you, you resist them actively or attack them.
The teenage attackers fled when the two men fought back...
We should take some comfort from the ability of the judicial system to fight back against corruption.
PHRASAL VERB : V P , V P against n
2.
If you fight back an emotion or a desire, you try very hard not to feel it, show it, or act on it.
She fought back the tears.
PHRASAL VERB : V P n (not pron) , also V n P