1.Originally referred to the duration of an amphetamine high. If it lasted a long time it had long legs. If it lasted a short while it had short legs. If it didn't get you high it didn't have any legs at all. 2. By extension in the mainstream it means how long anything lasts.
1. "The speed didn't have any rush but it had good legs."
2. "His career with the Dodgers had good legs. It lasted 17 years."
1. Originally when a speed dealer buys more speed to sell. (speed = an upper.)
2. Recently crept into the mainstream meaning anything extended or renegotiated, like a contract or term of office
1. "I can't sell you an 8-ball until after i re-up."
2. "Shaquille O'Neal re-upped his contract with the Lakers."