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.