A.T. Robertson 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)

When they heard (ακουοντες). Present active participle of ακουω, while hearing.

They were cut to the heart (διεπριοντο ταις καρδιαις). See 5:33 where the same word and form (imperfect passive of διαπριω) is used of the effect of Peter's speech on the Sadducees. Here Stephen had sent a saw through the hearts of the Pharisees that rasped them to the bone.

They gnashed on him with their teeth (εβρυχον τους οδοντας επ' αυτον). Imperfect (inchoative) active of βρυχω (Attic βρυκω), to bite with loud noise, to grind or gnash the teeth. Literally, They began to gnash their teeth at (επ') him (just like a pack of hungry, snarling wolves). Stephen knew that it meant death for him.