Green’s Dictionary of Slang

high lawyer n.

a highwayman.

1592		Greene Blacke Bookes Messenger 13: I came to the credite of a high Lawyer, and with my sword freebooted abroad in the country like a Cauelier on horsebacke.
1600		J. Day Blind Beggar of Bednall-Green Act I: He wo’d be your prigger, your prancer, your high-lawyer.
1608		Dekker Belman of London G4: The theefe that commits the Robbery, and is chiefe Clearke to Saint Nicholas, is called the High Lawyer.
1610		Rowlands Martin Mark-all 50: Such as robbe on horse-backe were called high lawyers.