quoi dire
I was at a loss as to what to say.
Je ne savais plus quoi dire.
I've worked out what to say, how to.
J’ai trouvé quoi dire et comment le dire.
— The Legend of Bhagat Singh (2002)
Think well about to what to say and do.
Réfléchissez bien à ce que vous allez dire et faire.
— Investigation of a Citizen Above Suspicion (1970)
I didn't know what to say to him.
Je ne savais pas quoi lui dire.
Since I didn't know what to say, I remained silent.
Comme je ne savais pas quoi dire, je suis resté(e) silencieux(se).
I really don't know what to say.
Je ne sais vraiment pas quoi dire.
Not knowing what to say, I remained silent.
Ne sachant que dire, je suis resté silencieux.