The compound "third-class" is a straightforward combination of the ordinal number "third" and the noun "class," denoting a rank or category. It emerged in the 19th century, primarily in the context of transportation, to designate the lowest and cheapest category of passenger accommodation, following the established hierarchy of "first-class" and "second-class." The logic is purely sequential and hierarchical, where "third" indicates its position at the bottom of a graded system of quality, comfort, or status. This literal meaning of being the third and lowest rank was then extended metaphorically to describe anything considered inferior or of the poorest standard.

Structure: third + class - third: From Old English þridda, from Proto-Germanic \þridjô, from Proto-Indo-European \triti̯o-. A numeral, meaning "next after the second." - class: From Latin classis, originally "a division of the Roman people for the purpose of taxation," later "a group, rank, fleet." In English, it denotes a rank or grade of quality.