"not high, below the usual level," late 13c., earlier lah (late 12c.), "not rising much, being near the base or ground" (of objects or persons), also "lying on the ground or in a deep place" (late 13c.). This is not found in Old English, so the word is probably from Old Norse lagr "low, low-down, short; humble," or a similar Scandinavian source (compare Swedish låg, Danish lav), from Proto-Germanic *lega- "lying flat, low" (source also of Old Frisian lech, Middle Dutch lage, Dutch laag "low," dialectal German läge "flat"), from PIE root *legh- "to lie down, lay."
In reference to sounds, "not loud," also "having a deep pitch," from c. 1300. Meaning "humble in rank" is from c. 1200; "undignified, not high in character" is from 1550s; meaning "coarse, vulgar" is from 1759. Sense of "dejected, dispirited" is attested from 1737. Of prices, from c. 1400. In geographical usage, low refers to the part of a country near the sea-shore (c. 1300), as in Low Countries "Holland, Belgium, Luxembourg" (1540s). Low German languages (1845) are so called for being spoken in the lower elevations of old Germany.
Abject, low, and mean may have essentially the same meaning, but low is more often used with respect to nature, condition, or rank: mean, to character or conduct: abject, to spirit. [Century Dictionary, 1897]
Low blow in the figurative sense (1940s) is from pugilism. To lie low is from mid-13c. as "get down so as not to be seen," 1880 in the modern slang sense "keep quiet." Low Church in 18c. English history referred to Anglicans laying little stress on church authority (1702); in 19c. it meant evangelical Anglicans.