if you know what's what, you have a lot of experience and can judge people and situations well.

Harry's been in the business for 40 years - he knows what's what.

• a bum steer
• a bundle of joy
• a busman's holiday
• a buzz word
• a call girl
• a calling card
• a camp follower
• a can of worms
• a carbon copy
• a carrot and stick
• a carrot and stick approach
• a carrot-top
• a case in point
• a cash cow
• a cat and mouse game
• a cattle market
• a cause celebre
• accept as gospel
• accept as gospel truth
• accept something as gospel
• accept something as gospel truth
• accidentally on purpose
• accidents will happen
• account for
• account for something