Rampant

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Middle English (as a heraldic term): from Old French, literally ‘crawling’, present participle of ramper (see ramp). From the original use describing a wild animal arose the sense ‘fierce’, whence the current notion of ‘unrestrained’.

wiktionary

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From Middle English rampand, rampend, present participle of rampen(“to rise by climbing, shoot up, sprout, sty, ascend”), from Old French ramper(“to creep, climb”) (see below), equivalent to ramp +‎ -and or ramp +‎ -ant. Recorded since 1382, "standing on the hind legs" (as in heraldry), later, "fierce, ravenous" (1387). Compare Scots rampand(“rampant”).

Alternatively from Middle English *rampant(not found), from Old French rampant, the present participle of ramper(“to creep, climb”), equivalent to ramp +‎ -ant. Old French ramper derives from Frankish *rampōn, *hrampōn(“to hook, grapple, climb”), from *rampa, *hrampa(“hook, claw, talon”), from Proto-Germanic *hrempaną(“to curve, shrivel, shrink, wrinkle”). More at ramp.

etymonline

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rampant (adj.)

c. 1300, raumpaunt, "standing on the hind legs" (as a heraldic lion often does), thus, also, "fierce, ravenous" (late 14c.), from Old French rampant, rampans, present participle of ramper "to climb, scale, mount" (see ramp (v.)). Sense of "growing without check" (in running rampant), is recorded by 1610s, probably is via the notion of "fierce disposition" or else preserves the older French sense. Related: Rampantly.