Why is "pound" (of weight) abbreviated "lb"?

Answers to Correct usage of lbs. as in "pounds" of weight suggest that "lb" is for "libra" (Latin), but how has this apparent inconsistency between the specific unit of weight "pound" and the more general concept of weight "libra" arisen? It seems analogous to abbreviating "grams" to "w" for weight which makes no sense to me.

Update: very nice historical context provided in your answers. Interesting that the Latin term has evolved into both pound (weight) in English and liter (volume) through Greek. I wonder if this is related to how "ounce" can refer to both weight and volume (fluid ounce)?

Lībra in Latin originally means ‘stone’, thence ‘pound weight’ (i.e., the little stone you put on scales to weigh things), thence ‘pound’ (the weight of one of those stones), and only from that was the meaning generalised to mean ‘weight’ in general. The phrase lībra pondō ‘a pound by weight’ was then used to refer unambiguously to the second meaning.

The second word in that phrase, pondus ‘weight’, was borrowed from Latin into some stage of Common Germanic at some point, but in the narrowed sense of lībra pondō—i.e., cutting off the ‘wrong’ half of the sentence. This is then the source of the English word ‘pound’. Similarly, an earlier, Proto-Italic, form of the word lībra (*līðra) was borrowed into Greek, where it became used as a specific measure of liquids: λίτρον/λίτρα ‘litre’.

This is quite similar to how ‘kilo’ (meaning ‘thousand’) has been generalised to a kilogram (a thousand grams), except the latter is a much later development. Or to how ‘mile’ (meaning also ‘thousand’) has come to mean a specific measure of length (originally mīlle passus ‘a thousand paces’ or mīlia passuum ‘thousands of paces’).

It is very common for generic units of measurements to become fixed in meaning to a specific measure of that unit; and also for determiners of measurements to become fixed in meaning to the measurement they are determining.

Related to this question is the fact that another symbol was also originally derived from the word libra.

This is the symbol £, which is an ornate letter "L" (from libra), now used to denote the Pound sterling, more commonly referred to as the British pound (ISO code GBP).

Prior to 'decimalisation' in February 1971, British currency followed the structure that had been used since pre-800 AD. (Further details in this answer.) The Pound sterling, being the main currency unit, continued unchanged from pre-decimalisation into the current decimalised currency.

The name Pound and the symbol £, originate from the fact that the value of the Anglo-Saxon pound was equivalent to one pound weight (libra) of silver.

The association of the word sterling with the British pound originates from shortly after the Norman Conquest of England in 1066, but the actual etymology is unclear. Subsequently, the composition of the silver used to make the coinage was specified by King Henry II, in 1158, as 92.5% pure silver, which subsequently became known as Sterling silver.

The pound sterling is the world's oldest currency still in use.

This isn't really about English, but Latin. However, since Latin.SE hasn't got off the ground yet, here is as good a place as any to answer.

Librum is Latin for 'a pound in weight'. It is also Latin for 'a pound weight' (that you put on one side of the scales), and 'weight' as an abstract concept. (The three are closely connected in all languages; whether the Romans actually distinguished between them would be an interesting question for Latin.SE.) Neither of the latter two words really need an abbreviation, but the third clearly does, so lb. is generally understood as standing for 'pounds avoirdupois'.

Libra (both singular and plural, and abbreviated lb), was a standard of weight used by ancient Romans. The Latin word may encompass the English for "pound," "scales," or "weights".

See:
- http://dictionary.reference.com/browse/libra
- http://translate.google.ca/?hl=en&tab=wT#la/en/libra