A form of pipto; to trip, i.e. (figuratively) to err, sin, fail (of salvation) -- fall, offend, stumble.
see GREEK pipto
1. transitive, τινα, to cause one to stumble or fall.
2. intransitive, to stumble: δίς πρός τόν αὐτόν λίθον, Polybius 31, 19, 5. tropically (cf. English trip, stumble)
a. to err, to make a mistake (Plato, Theact c. 15, p. 160 d.); to sin: absolutely Romans 11:11 (ἴδιον ἀνθρώπου φιλεῖν καί τούς πταιοντας, Antoninus 7, 22); πολλά, in many ways, James 3:2; ἐν ἑνί (namely, νόμῳ), to stumble in, i. e. sin against, one law, James 2:10 (but see εἷς, 2 a. at the end); ἐν λόγῳ (for the (more common) simple dative), to sin in word or speech, James 3:2.
b. to fall into misery, become wretched (often so in Greek writings): of the loss of salvation, 2 Peter 1:10.