While true that originally it also meant "balance" (in late latin we later got the word bilanx for scales - literally two plates), lb is for latin libra, and it was an actual specific weight (with a corresponding economical value in the Roman as coin) in use since the Roman republic, and it has remained in use for more than two millennia all over the world (with slighly different values depending on time and place, but usually between 350-450grams). It's not an accident the the UK pound stirling coin still has the same L shaped symbol £, same as the italian lira, etc

How ironic that Americans use a foreign language to describe their unit of measurement that is used nowhere outside the USA.