Lb is an abbreviation of the Latin word libra. The primary meaning of libra was balance or scales (as in the astrological sign), but it also stood for the ancient Roman unit of measure libra pondo, meaning “a pound by weight.” We got the word “pound” in English from the pondo part of the libra pondo but our abbreviation comes from the libra. The libra is also why the symbol for the British pound is £—an L with a line through it. The Italian lira also used that symbol (with two lines through it), the word “lira” itself being a shortened version of libra.
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I always wondered about that.