Two jobs, one suffix
In current English, ruler names two everyday things. One is a person who governs, especially a sovereign. The other is a rigid, usually marked strip of wood, metal, or plastic used to draw a straight line or to measure length. Speakers often treat the pair as an accident of spelling. They are one word, formed the same way, from a Latin picture in which a straight stick is also a standard.
Formed in English, not borrowed whole
Ruler is an English derivation. Middle English has reuler, built from rule plus the native agent suffix -er. It did not arrive as a ready-made French or Latin agent noun. What English borrowed earlier was the base; the -er form is home-grown.
Careful dating puts the noun in the fourteenth century. The Oxford English Dictionary’s earliest evidence is before 1382, in the Early Version of the Wycliffite Bible, already in the sense of someone who governs. Collins gives a wider Middle English window, 1325–75, a period label rather than a single citation. Merriam-Webster simply says the fourteenth century. The documented first use is political, not classroom.
So the origin of the word ruler is English morphology sitting on a Romance base. The suffix is the ordinary -er of baker, singer, and ruler of a realm alike.
The base: a French rule from a Latin stick
Rule is older than ruler by about two centuries. Around 1200 English took Old French riule and Norman reule, words for a principle of conduct, a custom, and a religious order. Modern French later partly re-Latinized the noun as règle. Those French forms go back to Vulgar Latin *regula and to Classical Latin rēgula: a straight stick, a bar, a rod used to keep a line true. Figuratively the same Latin noun meant a pattern or model.
Latin already held both pictures that English later split across rule and ruler. Cicero uses rēgula for the carpenter’s or draftsman’s tool, alongside the plumb line and the square, and also for a measure of speech, law, or philosophy. The verb beside it is regere, to guide, straighten, or govern; the related noun rēx is king. An instrument noun on that verb is a natural Latin formation: the thing with which you keep a line straight.
English rule as a physical strip, the unmarked older name for a drawing or measuring stick, is in use by the mid-fourteenth century. Ruler as the name of that instrument follows by about 1400. For a time the two nouns competed for the tool; rule is still available in that sense, especially in technical English, while everyday speech has largely given the stick to ruler.
Spanish regla, Italian regola, and German Regel (a Latin loan into Germanic) keep the same family in view. They are siblings of English rule, not descendants of ruler.
A root that means a straight line
Behind Latin lies a Proto-Indo-European root reconstructed as *reg- or, with the laryngeal written in, *h₃reǵ-. The core idea is to move in a straight line, then to direct along that line, then to lead. That is why a measuring stick and a sovereign sit in one family. Directing a line and directing people are the same metaphor at the root, not a later joke.
Cognates make the map plain. Sanskrit rāj- and rāja, Old Irish rí, Gothic reiks, and Latin rēx are all king-words from that root. English right, rich, regal, regular, regulate, rector, and region grow from the same stretch-and-direct idea, some through Germanic, some through Latin and French. None of them is a child of ruler; they are cousins that show why the English noun could mean both a monarch and a marked strip without a folk-etymology leap.
The English suffix is transparent. -er marks the person who rules. The same suffix commonly names instruments, so the device that rules lines is not a second invention. Merriam-Webster still records a narrower sense: a worker or a machine that rules paper. A later school verb, to ruler someone, meaning to strike with the stick, is formed from the tool, not from kingship.
How the senses moved
The political sense comes first in the surviving record: a governor, especially one with superior or sovereign authority. The drawing instrument is only a generation later. English already had rule for the stick, so ruler was at first a competitor rather than a replacement. American usage often narrows ruler to a one-foot strip, set against yardstick and meter stick; that is a habit of measurement systems, not a change in etymology.
The verb rule kept growing on its own. Courts rule in the sense of laying down a decision from the early fifteenth century. Pages are ruled with parallel lines from the 1590s. Those developments feed the paper-ruling trade and the classroom object, but they do not create a new root. The two modern nouns never left the Latin picture of a straight standard.
Old English had already borrowed Latin rēgula as regol, mainly for a monastic rule of life, and formed regolsticca for a measuring rod. Middle English rule absorbed both the French loan and that older English Latinism. Ruler, though, is the later -er coinage. It is not a continuation of regolsticca, and it is not a separate borrowing of Latin rēgula. The stick-word that English inherited twice, once from Old English and once from French, is rule. Ruler is what English did with that verb and noun once it wanted an agent.
Sources do not disagree on the path in any serious way. They differ on how tightly to date the first citation. A broad Middle English range and a specific Wycliffe date can both be true if the former is a period and the latter is the earliest surviving text. There is no evidence that the measuring sense is the original English meaning, and no evidence that the political sense is a metaphor piled onto a school tool. If anything, the Latin object is the older of the two pictures, and English happened to record the person first.
What the stick still shows
Keep one concrete fact. Latin rēgula named the rod, then the standard. English built ruler on the verb and noun that came from that rod, and within a few decades the new agent noun covered both the person who directs and the stick that draws a line. The school ruler is not a nickname for a king. It is the older object, still doing the job the Latin word started with.