Meaning

adverb within a building

noun the region that is inside of something

adjective relating to or being on the side closer to the center or within a defined space

originFrom Middle English ynneside; equivalent to in- + side. Compare German Innenseite (“inside”), Danish inderside (“inside”), Swedish insida (“inside”), Dutch binnenzijde (“inside”), German Low German Binnensied, Binnersied (“inside”), Saterland Frisian Binnersiede (“inside”).

Etymology from Wiktionary, CC BY-SA 3.0