Is there a medieval term for a highwayman?
Solution 1:
In decreasing likelihood, I might choose
"brigand" ‒ 14th Century
"bandit" ‒ 1611
"robber" ‒ 13th Century or
"thug" ‒ 1810.
Solution 2:
The word "Cutpurse" is one of my personal favorites. The origins of that word fit snugly into the tail end of your timeframe.