A molly ball (or mollyball, one word) is the pure form of the chemical MDMA (commonly referred to as ecstasy) which has been rolled into the center of a ball of some type of food and then eaten. This may include, but is certainly not limited to:
Someone or something that bites your ankles.
To a postman, an ankle biter is often known as a dog.
To an adult, an ankle biter may be a toddler.
To hikers, an ankle biter is sometimes a tick.
And so on.