also featherweight, "lightest weight allowable by rules," 1812 (earlier as simply feather, 1760), from feather (n.) + weight (n.). Originally in horse-racing; boxing use as a specific weight class dates from 1889.
Old English feðer "a feather; a pen," in plural, "wings," from Proto-Germanic *fethro (source also of Old Saxon fethara, Old Norse fioþr, Swedish fjäder, Middle Dutch vedere, Dutch veder, Old High German fedara, German Feder), from PIE *pet-ra-, from root *pet- "to rush, to fly."
Feather-headed "silly" is from 1640s. Feather-duster attested by 1835. Figurative use of feather in (one's) cap attested by 1734. Birds of a feather "creatures of the same kind" is from 1580s; the same image is in Greek homopteros (variant birds of a beak is from c. 1600).
Old English gewiht "weighing, weight, downward force of a body, 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).
Figurative sense of "burden" is late 14c. To lose weight "get thinner" is recorded from 1961. Weight Watcher as a trademark name dates from 1960. To pull one's weight (1921) is from rowing. To throw (one's) weight around figuratively is by 1922. Weight-training is from 1945. Weight-lifting is from 1885; weight-lifter (human) from 1893.