An "Air" sandwich is a food product made of nothing at all. Normally it's consumed without the buns. Therefore, the eater is actually eating nothing at all.
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.