Origins
The Latin word 'rēx' — meaning 'king' — carries within it one of the deepest metaphors in human political thought: that to rule is to make straight, to set right, to direct. The word descends from the Proto-Indo-European root *h₃reǵ-, meaning 'to straighten' or 'to direct,' and its etymological family encompasses not only kingship and royalty but also the entire conceptual domain of straightness, correctness, direction, and regulation.
The PIE root *h₃reǵ- produced king-words across the Indo-European family. Sanskrit rājan ('king') is the source of 'rajah' and 'maharajah' in English. Old Irish rí ('king,' genitive ríg) appears in countless Irish place names and personal names. Gothic reiks ('ruler') was borrowed into Finnish as rikas ('rich') and is the ancestor of German Reich ('realm, empire'). Through Proto-Germanic *rīkijaz ('powerful'), the root also gave English 'rich' — a word that originally meant 'powerful, mighty, of high rank' before narrowing to its modern financial sense.
In Latin, rēx was a third-declension noun (stem rēg-). After the Romans expelled their last king, Tarquinius Superbus, in 509 BCE, rēx became a politically toxic word in Roman culture. To call someone a rēx was to accuse them of tyranny. Julius Caesar was assassinated in part because his enemies believed he aspired to become rēx. This Republican horror of kingship profoundly influenced the American founding fathers, who knew their Roman history intimately.
Latin Roots
The related Latin verb regere ('to direct, to guide, to rule') and its past participle rēctus ('straight, right, correct') generated most of the enormous English word family. From regere come 'regent' (one who rules), 'regime' (a system of rule), 'regiment' (originally a system of governance, then a military unit under governance), and 'region' (an area under direction or rule). 'Regular' comes from rēgula ('a straight stick, a rule, a pattern'), the diminutive of rēctus.
From rēctus ('straight') come 'rectangle' (straight-angled), 'rectify' (to make straight), 'rector' (one who directs), 'correct' (thoroughly straight), 'direct' (thoroughly guided), and 'erect' (set upright). The verb 'dress' comes ultimately from Latin dīrigere (from dis- + regere, 'to arrange, to set straight'), through Old French dresser — to dress originally meant to set straight, to arrange, and the modern senses of clothing and wound treatment both derive from this straightening metaphor. 'Address' is ad- + dress ('to direct toward').
Through Old French, rēx produced the most common English words for kingship. 'Royal' descends from Old French roial, from Vulgar Latin *rēgālis. 'Regal' was borrowed later, directly from Latin rēgālis, creating a doublet pair. 'Reign' comes from Old French reigne, from Latin rēgnum ('kingdom, royal power'). 'Realm' comes from Old French reialme, from a Vulgar Latin blend of *rēgālimen and rēgnum.
Proto-Indo-European Roots
The deep metaphor of *h₃reǵ- — that to rule is to straighten — reflects a Bronze Age conception of leadership as the imposition of order on chaos, the drawing of straight lines through crooked territory. A 'ruler' is both a sovereign and a straight-edge for drawing lines. To 'regulate' is to bring under a rule. To 'correct' is to make thoroughly straight. A 'regime' is a system that keeps things in line. This metaphor pervades English so thoroughly that it is invisible: we speak of 'right' and 'wrong,' 'correct' and 'incorrect,' 'regular' and 'irregular,' without sensing the ancient equation between moral rectitude and physical straightness.
The legal and ceremonial uses of rēx persist in English. British legal cases are titled 'R v. [defendant]' — R standing for Rex or Regina depending on the monarch's gender. 'Regalia' (from Latin rēgālis) denotes the emblems and insignia of royalty. The dinosaur Tyrannosaurus rex ('tyrant lizard king'), named in 1905, brought the Latin king-word into popular culture with a vividness that no etymology textbook could match.
The reach of *h₃reǵ- into modern English is extraordinary. 'Surge' (from sub- + regere, 'to rise up from below'), 'source' (from surgere via Old French sourse), 'resource' (a rising again), 'insurrection' (a rising up against), and 'resurrection' (a rising again) all trace back to the same root through the Latin compound surgere. From straightening to ruling to rising, the PIE concept of directional force has shaped the English vocabulary of power, order, and governance for six thousand years.