Green’s Dictionary of Slang

padder n.1

[pad n.1 (1); Rowlands, Martin-Mark-all (1610), differentiates between the types of highway thief: ‘Such as robbe on horse-backe were called high lawyers, and those who robbed on foote...called Padders’ ]

[17C–mid-18C] one who robs on the highway, but does not work from a horse.