C13: from Old French asalir, from Vulgar Latin assalīre (unattested) to leap upon, from Latin assilīre, from salīre to leap
Word Origin and History for assail
v.
c.1200, from Old French assalir "attack, assault, assail" (12c., Modern French assaillir), from Vulgar Latin *adsalire "to leap at," from Latin ad- "at" (see ad-) + salire "to leap" (see salient (adj.)). Figurative use from mid-14c. Related: Assailed; assailing; assailable.