Origin and history of middleweight

Old English middel, "equally distant from extremes or limits; intermediate," from Proto-West Germanic *midla- (source also of Old Frisian middel, Old Saxon middil, Middle Low German, Dutch middel, Old High German mittil, German mittel), from Proto-Germanic *medj, from PIE root *medhyo- "middle."

Middle finger "the third finger" (counting the thumb as the first) so called from late Old English. Middle school is attested from 1838, originally "middle-class school, school for middle-class children;" the sense in reference to a school for grades between elementary and high school is from 1960. Middle management, the level below senior management, is by 1941.

Middle-of-the-road in the figurative sense is attested from 1894, originally political; edges of a dirt road can be washed out and thus are less safe, but the notion here probably is of the middle as "less exposed to ambush." Middle way in the figurative sense of "path of moderation" is from c. 1200. Middle ground as "place of moderation or compromise between extremes" is by 1961. Middle-sized "of medium size" is by 1620s.

In U.S. history, the Middle States (1784) were those between New England and the South (New York, New Jersey, Pennsylvania, Delaware). Middle America for "the 'silent majority,' the generally conservative middle class regarded as a homogeneous group" is by 1968.

Middle English weght, from Old English wiht, gewiht "weighing, downward force of a body, physical property of heaviness," from Proto-Germanic *wihti- (source also of Old Norse vætt, Danish vegt, Old Frisian wicht, Middle Dutch gewicht, German Gewicht), from *weg- (see weigh).

By mid-14c. as "piece of metal used on account of its weight" (fishing-weight, etc.). The figurative sense of "burden" is late 14c. In reference to the weights of a clock by 1510s.

By late 14c. as "what one weighs;" to lose weight "get thinner," of persons, is recorded by 1858. Weight Watcher as a trademark name dates from 1960.

The sense of "importance" is by 1520s. To throw (one's) weight around figuratively is by 1922. Weight-training is from 1945. Weight-lifting is from 1885; to lift weights for exercise or muscle development is by 1907; weight-lifter (human) is from 1893.

To pull one's weight was prominent from 1902, a phrase of U.S. President Theodore Roosevelt that caught the attention of political journalists.

The first requisite of a good citizen in this republic of ours is that he shall be able to pull his weight—that he shall not be a mere passenger, but shall do his share in the work that each generation of us finds ready to hand .... [extract from Roosevelt speech at New York Chamber of Commerce, printed in Lincoln (Neb.) Journal Star, Nov., 17, 1902]

The image is from rowing, where it is attested by 1869.

A shade heavy he may be ; but he pulls his weight many times over. So tremendous, indeed, is his muscle, that, if he were to lash out, he would pull the University eight right under water (etc.) [political humor masked as an account of a rowing race, Cheshire Observer, March 20, 1869]