(to be) a tall order noun phrase informal
Used to refer to a request, task, or job that is very difficult to carry out
The task my boss has just given me is certainly a tall order.
He already knows it's a high order, but he still accepts it.
Raising the production to be 6.5 million barrels per day is a tall order indeed.
Used to refer to a person who is your enemy
He faced his sworn enemy.