The origin is in the Latin word
libra, which could mean both balance scales (hence the symbol for the astrological sign Libra, which was named after a constellation that was thought to resemble scales) and also a pound weight, for which the full expression was
libra pondo, the second word being the origin of our
pound.
Also, if pound was abbreviated as pd, it would stand for per day. So instead of saying that is 2 pounds abbreviated as pd, you would be saying that is 2 per day.