Reading these words as onomatopoetic gives a slightly different nuance to each of the four verses in the New Testament where they are used.
Reading this as the less intense, "startled", "taken by surprise" gives greater coherence with the following information. His listeners are now expecting that these things will happen, so it should come as no surprise to them when they do.Luke 21:9 wrote:Ὅταν δὲ ἀκούσητε πολέμους καὶ ἀκαταστασίας, μὴ πτοηθῆτε· δεῖ γὰρ ταῦτα γενέσθαι πρῶτον, ἀλλ’ οὐκ εὐθέως τὸ τέλος.
Here reading this as closer to the startling that produces the verbal reaction, the natural progress of emotion is better expressed. "They got startled, then began to feel fear in their hearts". Somebody first gets a startle then, fear begins to grow, as rationality finds the thing initially seen as contrary to nature.Lk 24:37 wrote:Πτοηθέντες δὲ καὶ ἔμφοβοι γενόμενοι ἐδόκουν πνεῦμα θεωρεῖν.
Reading this as onomatopoetic rather than a deeper emotion of the heart, the impulsive nature of emotions is given greater emphasis. A softer meaning of almost "don't be taken by surprise by anything (done) by those who oppose (the Gospel)" is possible.Philippians 1:27,28 wrote:Μόνον ἀξίως τοῦ εὐαγγελίου τοῦ χριστοῦ πολιτεύεσθε, ἵνα εἴτε ἐλθὼν καὶ ἰδὼν ὑμᾶς, εἴτε ἀπών, ἀκούσω τὰ περὶ ὑμῶν, ὅτι στήκετε ἐν ἑνὶ πνεύματι, μιᾷ ψυχῇ συναθλοῦντες τῇ πίστει τοῦ εὐαγγελίου, καὶ μὴ πτυρόμενοι ἐν μηδενὶ ὑπὸ τῶν ἀντικειμένων· ἥτις αὐτοῖς μέν ἐστιν ἔνδειξις ἀπωλείας, ὑμῖν δὲ σωτηρίας, καὶ τοῦτο ἀπὸ θεοῦ·
Here, I think this is a variation on the same construction of meaning as in Luke above. Reading the πτόησις as onomatopoetic, ie. retaining a close relationship between the drawing or exhalation of breath and the the startling experience, the meaning is something like, "not be driven to fear by every little thing that startles you".1 Peter 3:6 wrote:ὡς Σάρρα ὑπήκουσεν τῷ Ἀβραάμ, κύριον αὐτὸν καλοῦσα, ἧς ἐγενήθητε τέκνα, ἀγαθοποιοῦσαι καὶ μὴ φοβούμεναι μηδεμίαν πτόησιν.
Reading πτοέω as a vaguely defined "fear" is quite inadequate and open to a wide variation in interpretation. Reading it with the connotations of onomatopoetic interpretation gives greater clarity to the meaning.