Definition of "rampant"

‘I forget your coat of arms.’‘A human foot d’or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel.’

1846, Edgar Allan Poe, The Cask of Amontillado

little pieces of moustache on his upper lip, like a pair of minnows rampant

1892, Thomas Hardy, The Well-Beloved

Conditions were horrendous aboard most British naval vessels at the time. Scurvy and other diseases ran rampant, killing more seamen each year than all other causes combined, including combat.

2012 March, William E. Carter, Merri Sue Carter, “The British Longitude Act Reconsidered”, in American Scientist‎, volume 100, number 2, page 87

In contrast to the despair of his opposite number, it was a day of delight for new City boss Manuel Pellegrini as he watched the rampant Blues make a powerful statement about their Premier League ambitions.

2013 September 22, Phil McNulty, “Man City 4-1 Man Utd”, in BBC Sport