rampant (adj.)

late 14c., "standing on the hind legs" (as a heraldic lion often does), thus, also, "fierce, ravenous" (late 14c.), from Old French rampant, present participle of ramper "to climb, scale, mount" (see rampage (v.)). Sense of "growing without check" (in running rampant), first recorded 1610s, probably is via the notion of "fierce disposition" or else preserves the older French sense.