• The Romans had a unit of weight called the "Libra" which was equal to about 3/4 of a modern pound, or 12 ounces. The first Britsh "pound" was equal to the Libra, hence the abbreviation Lb.
• The Latin words libra pondo describe a Roman unit of weight similar to a pound. The abbreviation "lb" for a unit of weight is derived from the Latin word libra. The word "pound" itself comes from the Latin pendere, to weigh, while libra meant “scales and balances” “lb” is singular (one pound) and “lbs” is plural as in pounds of weight.
• Strictly speaking, the use of 'lbs' as a contraction for pounds is incorrect. The correct term is 'lb' and it is applied whether one measures 0, 23, or -92 pounds. This is true of any term, for example: 'ft' (foot, feet), 'kg' (kilogram, kilograms), and 'N' (newton, newtons). At the very least, this avoids confusion with the symbol for seconds, 's'. The term 'ms^-2' means 'metres per second sqared', not 'one over metres (plural) squared'.
• A pound is always written as "lb" to prevent confusion with pound (money) It is very old, traced back to the Roman "libra" (which explains its abreviation!). It was defined in England since Ethelred the Unready (968-1016). In fact, a pound (money) was originally a pound (weight) of silver, and the symbol for pound (money) is a stylised L.