late 14c., "standing on the hind legs" (as a heraldic lion often does), thus, also, "fierce, ravenous" (late 14c.), from O.Fr. rampant, prp. of ramper "to creep, climb" (see rampage). Sense of "growing without check" (in running rampant), first recorded 1610s, preserves the O.Fr. sense.
