: to be very common or to spread very quickly and in a way that is difficult to control Rumors of her engagement ran rampant.

Is run rampant a saying?

(idiomatic) To go unchecked or without control; to be wild or excessive.

What is rampant?

1a : rearing upon the hind legs with forelegs extended. b : standing on one hind foot with one foreleg raised above the other and the head in profile —used of a heraldic animal. 2a : marked by a menacing wildness, extravagance, or absence of restraint rampant rumors.