A tall order is a request that will be very difficult to meet. That is a tall order, quite a tall order, a tall order in the time.
It is a polite way of saying no, or nearly no, since calling something a tall order concedes the request while doubting the outcome. Tall here is the old sense meaning large.
1nountall orderA request that will be very difficult to meet.
“Finishing the whole report by tomorrow morning is a tall order, but I'll try.”
tall order is a request.
Tall in the old sense of large plus order in the sense of a request.
Built from
From tall (“big, large”) + order (“request”).
Etymology from Wiktionary, CC BY-SA
Practised in Idioms