The Greek ekpiptō means to fall out of or fall away from — to be dislodged from one's position. It is used of stars falling from heaven (Mark 13:25; Revelation 6:13), of the angel Gabriel's visit when Zechariah's fear 'fell upon' him (Luke 1:12), and most significantly in Galatians 5:4 — 'You who are trying to be justified by the law have been alienated from Christ; you have fallen away from grace.' Also Acts 27:17, 26, 29 of ships running aground.
Galatians 5:4's use of ekpiptō from grace is one of Paul's sharpest warnings. To seek justification by law-keeping is not merely inadequate — it is to fall out of the entire sphere of grace. Grace and self-justification cannot coexist. Paul's point is not that believers can lose their salvation through sin, but that the very orientation toward law-righteousness places one outside the domain of grace. 2 Peter 3:17 uses ekpiptō similarly — beware lest you 'fall from your secure position' by being carried away with error. Stability in grace requires active attentiveness.