From Latin 'impetere' (to rush at), from 'in-' (toward) + 'petere' (to seek) — literally an attack or rushing forward.
The force or energy with which something moves; the stimulus or motivation behind an action.
From Latin 'impetus' (attack, assault, rush, violent onset), a noun from 'impetere' (to attack, to rush at, to assail), composed of 'in-' (toward, upon) + 'petere' (to go toward, to seek, to aim at, to attack). The PIE root is *peth₂- (to fly, to fall, to rush toward) — a root that connected the motion of birds, falling, and directed rushing. It surfaces in Greek 'petomai' (I fly) and 'pteron' (wing, feather), in English
The same root 'petere' gave English 'petition,' 'compete,' 'appetite,' and 'perpetual.'