Green’s Dictionary of Slang

legs n.

[i.e. all legs, no body]

1. a tall, thin person.

[UK]Leaves from Diary of Celebrated Burglar 62/2: Harry Bryan, who, owing to the extreme length of his propellers, carried the alias of Legs.
[US]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.

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