The compound "third-story" is a straightforward numerical-noun construction in English. The word "third" descends from the Proto-Indo-European root for "three," evolving through Germanic into its ordinal form. The more intriguing element is "story," which originates from the Latin historia, meaning "history" or "narrative." In medieval architecture, buildings, particularly churches, often had tiers of windows decorated with narrative paintings or sculptures depicting historical or biblical tales. This association led to each such tier being called a "history" or "story," a meaning that eventually generalized to any floor level within a building. Thus, a "third-story" room literally means a room on the third tier or level, a meaning that evolved from the concept of a level displaying pictorial stories.

Structure: third (number) + story (level) - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tritiyós. Functions as a numerical ordinal in the compound. - story: From Anglo-Latin historia (meaning "picture, story, floor"), from Latin historia ("history, tale"). In Medieval Latin, historia came to mean a tier of painted windows or sculptures on a building facade, which later transferred to mean the floor level itself. Functions as the noun denoting a floor or level in a building.