Origins
Regulate entered English in the early 17th century from Late Latin 'rēgulāre' (to direct, to control according to a rule), itself from the noun 'rēgula' (a straight stick, a ruler, a rule, a pattern). The underlying verb is 'regere' (to rule, to guide, to keep straight), from PIE *h₃reǵ- (to move in a straight line, to direct).
The PIE root *h₃reǵ- is one of the foundational roots of Western political vocabulary. Its core meaning — moving in a straight line, directing — extended naturally into governance: to rule is to keep things straight, to maintain order. From this single root came Latin 'rēx' (king), 'rēgīna' (queen), 'rēgnum' (kingdom), 'regiō' (direction, district, region), 'regere' (to rule), 'rectus' (straight, right), 'rector' (a guide, a ruler), and 'rēgula' (a straight stick, a rule).
The Latin noun 'rēgula' originally meant a physical object: a straight piece of wood used by builders and carpenters to ensure surfaces were flat and lines were true. This concrete tool became a metaphor for any standard or principle that keeps things in order. English 'rule' comes from the same word through Old French 'riule.' When we say 'ruler' to mean both a governing authority and a measuring stick, we are preserving both the abstract and concrete senses of 'rēgula.'
Figurative Development
The verb 'regulate' thus carries the deep metaphor of straightening — bringing things into line with a standard. Government regulation imposes rules to keep markets, industries, or behaviors within acceptable bounds. A regulator on a machine adjusts its operation to maintain steady performance. To regulate one's emotions is to keep them in order.
The word family radiating from 'regere' is enormous. 'Rector' (one who rules or directs) gave its name to heads of universities and parishes. 'Regime' (a system of government) came through French. 'Regent' (one who rules in place of a monarch) combines 'regere' with the present participle suffix. 'Region' was originally a direction or district under rule. 'Reign' reached English through Old French 'reigne' from Latin 'rēgnum.' Even 'royal' descends from 'regere' via 'rēgālis' (of a king) and Old French 'roial.'
In the Germanic branch, PIE *h₃reǵ- produced Old English 'riht' (straight, just), which became modern 'right.' The connection between straightness and justice — between keeping things in line and keeping things fair — runs through both the Latin and Germanic descendants of this ancient root.
Legacy
Modern debates about regulation — how much government control is appropriate, where to draw the line between freedom and order — echo the fundamental tension built into the word itself. To regulate is to straighten, to correct, to impose a standard. The question societies perpetually face is how straight the line needs to be.