something that you say in order to show you think an event or action is not likely to happen.

A pay rise? That'll be the day!

• big-headed
• big in
• big in something
• big-mouth
• big mouth
• big noise
• big on
• big on something
• big picture
• big shot
• big smoke
• big ticket
• big time
• big-time
• big wheel
• bill and coo
• billet-doux
• bird-brain
• bird-brained
• bird has flown.
• bird in the hand
• bird in the hand is worth two in the bush.
• birds and the bees
• bird's eye view
• birds of a feather