British & Australian, old-fashioned

to be crazy.

You went swimming in the sea in the middle of winter? You must be barking mad!

• a stiff upper lip
• a sting in the tail
• a stitch in time.
• a stitch in time saves nine.
• a stone's throw
• a stool pigeon
• a storm in a teacup
• as to the manner born
• as tough as nails
• a straight arrow
• a straight shooter
• a straw in the wind
• a stroke of luck
• a strong stomach
• a stud-muffin
• a stuffed shirt
• a stumbling block
• a sudden rush of blood
• a sudden rush of blood to the head
• as usual
• a sweet deal
• a sweet tooth
• as well
• as well as
• a sword of Damocles hangs over head