From Latin interior, the comparative form of inter (between), 'interior' belongs to a systematic Latin pattern that also produced 'exterior,' 'superior,' and 'inferior' — and whose superlative intimus gave us 'intimate.'
The inner part of something; the inside of a building, vehicle, or country.
From Latin interior (inner, comparative form), from inter (between, among), from PIE *h₁en (in). Latin formed comparative adjectives from prepositions: inter (between) became interior (more inward), and the superlative intimus gave English 'intimate.' The same pattern produced exterior from extra and superior from super. The word entered English in the late 15th century, initially
Latin built a neat system of comparatives from prepositions: inter (between) gave interior, extra (outside) gave exterior, super (above) gave superior, infra (below) gave inferior. The superlative of interior — intimus (innermost) — gave English 'intimate,' a word that literally means 'most inward,' explaining why it carries connotations of closeness and privacy.