Albert Barnes Commentary Matthew 26:74

"Then began he to curse and to swear, I know not the man. And straightway the cock crew." — Matthew 26:74 (ASV)

Then began he to curse, etc. Peter was now irritated beyond endurance. He could no longer resist the evidence that he was known. It had been repeatedly charged on him. His language had betrayed him, and there was a positive witness who had seen him.

He felt it necessary, therefore, to be even more determined. Accordingly, he added to the sin of denying his Lord the deep aggravation of profane cursing and swearing; affirming—what he must have known was false—that he did not know the man. Immediately then the cock crowed; that is, the second crowing, or not far from three in the morning.