The compound adjective 'third-tier' is a modern formation built from two distinct historical strands. 'Third' descends from a deep Proto-Indo-European root for the number three, which evolved through Germanic into the English ordinal. 'Tier' entered English from Old French, originally relating to order or sequence, and came to denote a level within a hierarchy. The logic of the compound is transparently numerical and hierarchical, directly combining 'third' and 'tier' to denote an entity positioned on the third level of a stratified system, typically implying a lower status or priority compared to the first and second tiers.

Structure: third (number) + tier (level) - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tritiyos. Functions as a numerical determiner. - tier: From Middle English tire, from Old French tire (sequence, order), likely from tirer (to draw, pull). Functions as a noun meaning a row, rank, or level.