junior (adj.)

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 is attested by 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]

junior (n.)

"a person younger than another; one of less experience or standing," 1520s, from junior (adj.). Generically as a name for a young boy, a young son from 1917, American English. In the U.S. college sense "student in the third year" from 1862.

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Definitions of junior from WordNet
junior (n.)
term of address for a disrespectful and annoying male;
look here, junior, it's none of your business
junior (n.)
a third-year undergraduate;
junior (n.)
the younger of two persons;
she is two years my junior
junior (adj.)
used of the third or next to final year in United States high school or college;
the junior class
Synonyms: third-year / next-to-last
junior (adj.)
younger; lower in rank; shorter in length of tenure or service;
junior (adj.)
including or intended for youthful persons;
junior fashions
a junior sports league
Junior (n.)
a son who has the same first name as his father;
Synonyms: Jr / Jnr