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.
second day of the week, Middle English monedai, from Old English mōndæg, contraction of mōnandæg "Monday," literally "day of the moon," from mona (genitive monan; see moon (n.)) + dæg (see day). A common Germanic name (compare Old Norse manandagr, Old Frisian monendei, Dutch maandag, German Montag). All are loan-translations of Late Latin Lunæ dies, which also is the source of the day name in Romance languages (French lundi, Italian lunedi, Spanish lunes), itself a loan-translation of Greek Selēnēs hēmera. The name for this day in Slavic tongues generally means "day after Sunday."
Yf cristemas day on A munday be,
Grete wynter þat yere ye shull see.
[proverb, c. 1500]
Phrase Monday morning quarterback is attested from 1932, Monday being the first day back at work after the weekend, where school and college football games played over the weekend were discussed. Black Monday (late 14c.) is the Monday after Easter day, though how it got its reputation for bad luck is a mystery (none of the usual explanation stories holds water). Saint Monday (1753) was "used with reference to the practice among workmen of being idle Monday, as a consequence of drunkenness on the Sunday" before [OED]. Clergymen, meanwhile, when indisposed complained of feeling Mondayish (1804) in reference to effects of Sunday's labors.
