Albert Barnes Commentary Acts 7:54

"Now when they heard these things, they were cut to the heart, and they gnashed on him with their teeth." — Acts 7:54 (ASV)

They were cut to the heart. They were exceedingly enraged and indignant. The whole course of the speech had been such as to excite their anger, and now they could restrain themselves no longer.

They gnashed on him, etc. Expressive of the bitterness and malignity.