The stone (abbreviation st) is a unit of measure equal to 14 pounds avoirdupois (about 6.35 kg [nb 1]) used in Great Britain and Ireland for measuring human body weight. In many Northwestern European countries the stone was formerly used for trade, with a value ranging from about 5 to 40 local pounds (3 to 15 kg).
If somebody is said to be once bitten twice shy, it means that someone who has been hurt or who has had something go wrong will be far more careful the next time. It’s something that you say when you have had an unpleasant experience and are going to be more careful to avoid a similar experience in the future.
The name derives from the location of the original Metropolitan Police headquarters at 4 Whitehall Place, which had a rear entrance on a street called Great Scotland Yard. The Scotland Yard entrance became the public entrance to the police station, and over time the street and the Metropolitan Police became synonymous.