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.
