Hang on to (someone or something) like grim death British verb phrase
To grip, grasp or hold someone or something tightly because you do not want to lose it or fall
As soon as the branch broke off, I hung on to another branch like grim death.
The verb "hang" should be conjugated according to its tense.
To decide to believe someone‘s saying although they may not tell the truth
He said that he was late because he had an accident and I gave him the benefit of the doubt.