What is a derogatory word or term for a peasant/lower-class person that is stronger or more insulting than pleb? It could be an archaic term used by nobles during the Middle Ages.
The first thing to grasp is that the nobility were generally not insulting of peasants. Many of them may have had little regard for them, treated them patronisingly (by today's standards), and certainly didn't want their daughters to marry one. But they had no need to be demeaning. Snobbery (as we know it today) is something that arrives with the emergence of the bourgeoisie from the mid-eighteenth century.
The English word peasant corresponds with the French paysan (fem -paysanne), which simply means 'country person'. In France country people are far more inclined to use the term to describe themselves. In modern English however the word 'peasant' is an insult enough in itself and in Britain can be applied to any person that the speaker doesn't like.
Plebeians were not peasants. In ancient Rome, they were an elevated order of 'free citizens' but lower than patricians. But using the term peasant or pleb to describe anyone in Britain today (I can't speak for America) is a considerable insult; as the British Cabinet Minister Andrew Mitchell discovered (though he claims he never said it).
Probably the expression Hoi polloi :
(Ancient Greek: οἱ πολλοί, hoi polloi, "the many"), is an expression from Greek that means the many or, in the strictest sense, the majority.
In English, it means the working class, commoners, the masses or common people in a derogatory sense.
Synonyms for hoi polloi, which also express the same or similar contempt for such people, include "the great unwashed", "the plebeians" or "plebs", "the rabble", "the dregs of society", riffraff", "the herd", "the proles" (proletariat) and "peons".
Ngram: ( Usage) the hoi polloi vs hoi polloi.
Mechanical and mechanic (literally, someone who works with their hands) were both used with derogatory implication (unlike plebian, serf, etc. which would be an insult only if applied to someone who was not of that class) relatively early. In Shakespeare's use in "A Midsummer Night's Dream" it's somewhere between a plain description of the people's class and an insult, (more snobbishly dismissive than insulting):
While she was in her dull, and sleeping hower,
A crew of patches, rude Mechanicals,
That worke for bread, vpon Athenian stalles,
Were met together to rehearse a play,
Intended for great Theseus nuptiall day:
But his use in "Antony & Cleopatra" is more directly contemptuous:
This is a Soldiers kisse: rebukeable,
And worthy shamefull checke it were, to stand
On more Mechanicke Complement, Ile leaue thee
And by the end of the 1600s it was more clearly an insult, to the extent that it hence died out in its non-insulting use (mechanicals would object to being called mechanicals) and then died out as an insult by consequence of people not knowing the original meaning. The final nail in the sense's coffin was the increase in the number of mechanics in the modern sense (though it does go back almost as far) with the Industrial Revolution.
It also has the advantage in terms of the question of being very much archaic today. It's not quite medieval, but that's impossible in Modern English, since English-speaking people then spoke Middle English, and if peyne þu to han an hauteyn speche, then nobody will understand it at all.
I can't find a linkable definition that doesn't require a login, but the OED has for mechanic this definition:
Belonging to or characteristic of the lower part of the social scale or the lower classes; vulgar, coarse. Now arch. and rare.
And for mechanic this definition:
Belonging to or characteristic of people engaged in manual work, esp. regarded as a class, artisanal; vulgar, coarse. Now rare.
There are a few others covering those who perform manual work, but without the derogatory nuance, since that came later.
Sadly, I can't remember where I had previously found versions of Johnson's and Webster's dictionaries. I can't remember if they define them so, but I do recall that Johnson using the word when he slighted the form smoothen as "A bad word among mechanicks for ſmooth", and Webster seemed to be copying that in saying it is "for smooth, is used by mechanics; though not, I believe, in the U. States." because I came across those when writing this answer.
An unsophisticated or socially awkward person from the countryside: she thought Tom a bit of a country bumpkin [Oxford Dictionaries Online]
Etymonline dates it back to the 16th century
"awkward country fellow," 1560s, probably from Middle Dutch bommekijn "little barrel," diminutive of boom "tree" (see beam (n.)). Apparently, though itself Dutch, it began as a derogatory reference to Dutch people as short and dumpy.
Also consider prole
a person who has low social status : a member of the working class [Merriam-Webster]
All of these terms have a negative or derogatory tone.
Villein in medieval England was not a derogatory term then but has since become villain with all its meanings of dishonesty and criminality.
early 14c., vileyn, spelling variant of villain in its reference to a feudal class of half-free peasants. It tends to keep the literal, historical sense of the word and let the pejorative meanings go with villain; Century Dictionary writes that "the forms villain, villein, etc., are historically one, and the attempt to differentiate them in meaning is idle," but Fowler finds this "a useful piece of differentiation." Related: Villeinage.
Yokel is a word commonly used, at least in the UK, to refer to anyone from the country, not sophisticated like city folk and considered a bit dim.
1812, perhaps from dialectal German Jokel, disparaging name for a farmer, originally diminutive of Jakob. Or perhaps from English yokel, dialectal name for "woodpecker."
Seems like there are two ideas mixed up here. "Peasant" (to me) means a rural person of low prestige and low income, though I've heard it used to refer to urban folks, too.
On the other hand "low class" could mean low income, low intelligence, low education, low morals, low standards of aesthetics, or many other kinds of "low". Peasants are low in a couple of these categories, but not all of them.
Picking the right word is difficult -- it depends which characteristic you want to use as the basis of your insult.
If you just want a general non-specific class-based put-down, then "scum" or "untouchable" might suit you. Neither is medieval, though, as far as I know.