Green’s Dictionary of Slang

1. a tall, thin person.

1865		Leaves from Diary of Celebrated Burglar 62/2: Harry Bryan, who, owing to the extreme length of his propellers, carried the alias of Legs.
1929		T. Wolfe Look Homeward, Angel (1930) 390: They’ll think Spring is here when they see old legs. They’ll need a stepladder to git at him.

2. speed.

2020

S.A. Crosby Blacktop Wasteland 104: ‘I bet she [i.e. an automobile] get up,’ Kelvin said. ‘Yeah. She got some legs’.