To deny or disown — to refuse to acknowledge a relationship or claim. Peter's three denials of Jesus are the word's most vivid NT use. It is the opposite of homologeo (G3670, confess/acknowledge). It can describe denying factual statements, denying one's faith under pressure, or denying oneself as a disciple.
Jesus's warning is stark: 'Whoever denies (arneomai) me before men, I will also deny before my Father who is in heaven' (Matt 10:33). The word therefore stakes everything on public acknowledgment of Christ. Yet Peter's restoration after his threefold denial (John 21) shows that denial is not the unforgivable sin — but it is a grave spiritual fall with profound consequences. 2 Timothy 2:12-13 uses both the possibility of denial and God's faithfulness in tension.