rampant

Concise Oxford English Dictionary © 2008 Oxford University Press:
rampant/ˈrampənt/

▶adjective

• 1 flourishing or spreading unchecked: rampant inflation.

• 2 unrestrained in action or performance.

• 3 [usu. postpos.] Heraldry (of an animal) represented standing on its left hind foot with its forefeet in the air.

– derivatives
rampancy noun,
rampantly adverb.

word history: Rampant entered Middle English from Old French, in which it meant ‘crawling’ (from ramper ‘to crawl’). As in French, early use in English had the adjective following the noun, especially in heraldry, as in ‘lion rampant’, in which the lion is depicted in profile, standing on its left hind foot, with forefeet and tail raised. By extension to other wild animals rearing up in a threatening manner, rampant acquired the sense ‘fierce’, from which developed the current meaning ‘unrestrained’.