The compound 'third-grader' is a transparent American English formation from the mid-20th century, directly combining the ordinal number 'third' with the agent noun 'grader'. 'Third' traces back through Old English to a Proto-Indo-European root meaning 'three', solidifying its role in sequence. 'Grader' derives from 'grade', which entered English via French from Latin gradus (step, degree), metaphorically applied to educational stages. The compound's logic is purely descriptive, literally meaning 'one in the third step/degree of schooling', reflecting the standardized, sequential structure of the U.S. education system.

Structure: third (ordinal number) + grader (one who is in a specified grade) - third: From Old English þridda, from Proto-Germanic \þridjô, from Proto-Indo-European \tritiyós. Functions as an ordinal number specifying a position in a sequence. - grader: From the base word grade (a level in a school curriculum, from Latin gradus 'step, degree') + the agent noun suffix -er (from Old English -ere, from Proto-Germanic \-ārijaz). Functions to denote a person associated with a specific grade.