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.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
<a href="https://www.etymonline.com/word/narrowness">Etymology of narrowness by etymonline</a>
Harper, D. (n.d.). Etymology of narrowness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/narrowness