Middle English narwe, from Old English nearu "of little width, not wide or broad; constricted, limited; petty; causing difficulty, oppressive; strict, severe," from West Germanic *narwaz "narrowness" (source also of Frisian nar, Old Saxon naru, Middle Dutch nare, Dutch naar) which is not found in other Germanic languages and is of unknown origin.
In reference to railroads, narrow-gauge (also narrow-gage) is by 1841, originally of those less than the standard of 4 feet 8 1/2 inches. The narrow seas (mid-15c.) were the waters between Great Britain and the continent and Ireland, but specifically the Strait of Dover.
Middle English narwen, from narrow (adj.) and in part from Old English nearwian "to force in, cramp, confine; become smaller, shrink." Related: Narrowed; narrowing.
c. 1200, nearewe "narrow part, place, or thing," from narrow (adj.). Old English nearu (n.) meant "danger, distress, difficulty," also "prison, hiding place."