early 15c., regulaten, "adjust by rule, method, or control," from Late Latin regulatus, past participle of regulare "to control by rule, direct," from Latin regula "rule, straight piece of wood" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").
Meaning "to govern by restriction" is from 1620s. Sense of "adjust (a clock, etc.) with reference to a standard of accuracy" is by 1660s. Related: Regulated; regulating.