This proverb derives from British history; because we are an island, ships and the navy have always been vital to us. When a ship is described as "sound," it indicates it has no leaks. So, being safe and sound implies being in a port with no damage to the ship, which is a welcome respite after a harrowing trip. Also, safe and sound is used to describe people who are not injured. This phrase is used in reference to those who have made it through unharmed from a dangerous situation or experience. Finally, this expression means that something is done so successfully that there was no need for help. For example, if you ask someone who does safe and sound activities how he/she is doing, they will answer you by saying "I'm safe and sound." In conclusion, safe and sound means successful without problems.