Entries linking to underweight
Old English under (prep.) "beneath, among, before, in the presence of, in subjection to, under the rule of, by means of," also, as an adverb, "beneath, below, underneath," expressing position with reference to that which is above, from Proto-Germanic *under- (source also of Old Frisian under, Dutch onder, Old High German untar, German unter, Old Norse undir, Gothic undar), from PIE *ndher- "under" (source also of Sanskrit adhah "below;" Avestan athara- "lower;" Latin infernus "lower," infra "below").
Productive as a prefix in Old English, as in German and Scandinavian (often forming words modeled on Latin ones in sub-). Notion of "inferior in rank, position, etc." was present in Old English. With reference to standards, "less than in age, price, value," etc., late 14c. As an adjective, "lower in position; lower in rank or degree" from 13c. Also used in Old English as a preposition meaning "between, among," as still in under these circumstances, etc. (though this may be an entirely separate root; see understand).
Under the weather "indisposed" is from 1810. Under the table is from 1913 in the sense of "very drunk," 1940s in sense of "illegal" (under-board "dishonest" is from c. 1600). To keep something under (one's) hat "secret" is from 1885; to have something under (one's) nose "in plain sight" is from 1540s; to speak under (one's) breath "in a low voice" is attested from 1832.
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.