late 13c., "younger, not as old as another," from Latin iunior "younger, more young," comparative of iuvenis "young; a young man," etymologically "one who possesses vital force," from PIE root *yeu- "vital force, youthful vigor" (see young (adj.)). Used after a person's name to mean "the younger of two" from late 13c. Abbreviation Jr. is attested from 1620s. Meaning "of lesser standing, more recent" is from 1766. That of "meant for younger people, of smaller size" is from 1860. Junior miss "young teenage girl" is from 1907. In U.S. colleges, "pertaining to the third-year." Junior college first attested 1896; junior high school is from 1909.
The junior high school is rapidly becoming the people's high school. The percentage of pupils completing the ninth year is constantly rising where junior high schools have been established. [Anne Laura McGregor, "Supervised Study in English for Junior High School Grades," New York, 1921]