The adverb form of pikros (bitter). Bitterly — with intense grief and remorse. Used of Peter's weeping after denying Jesus.
After denying Jesus three times, Peter went out and 'wept bitterly' (Matthew 26:75). This is one of the most poignant moments in the Gospels. Peter's bitter tears were not mere regret but the beginning of genuine repentance. Unlike Judas, whose remorse led to suicide, Peter's bitter weeping led to restoration. Bitter tears watered by grace produce sweet restoration.