Most of our abbreviations for units of measurements are pretty straightforward. They are made up of of letters from the words they stand for. So how do we get lb for "pound" and oz for "ounce"?
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.
"Ounce" is related to the Latin uncia, the name for both the Roman ounce and inch units of measurement. The word came into English from Anglo-Norman French, where it was unce or ounce, but the abbreviation was borrowed from Medieval Italian, where the word was onza. These days the Italian word is oncia, and the area once covered by the Roman Empire has long since switched to the metric system.