Why Are Pound and Ounce Abbreviated "lb." and "oz"...

“Lb.” stands for libra, the basic unit of Roman weight, from which our present-day pound derives. The libra weighed a little under 12 ounces avoirdupois.

“Oz.” stands for the Italian onza, ounce. It came into use in the 15th century. Ounce comes from the Latin uncia, a 12th, which is also the source of the term “inch.”