Ptýrō (πτύρω) means to terrify, to frighten, to intimidate. In the passive, 'to be terrified or alarmed.' Originally used of startled horses. It appears once in the NT.
Philippians 1:28: 'without being frightened in any way by those who oppose you.' Paul uses a word of panic — the kind that makes horses bolt — and tells the Philippians not to experience it when facing opposition. Courage in persecution is itself 'a sign to them that they will be destroyed, but that you will be saved — and that by God.' Fearlessness under pressure is theological testimony: it declares that God is sovereign, that death has been conquered, and that no opposition can separate us from Christ's love.