1801, "a story of a house," from Scottish flat "floor or story of a house," from Old English flett "a dwelling, hall; floor, ground," from Proto-Germanic *flatja-, from suffixed form of PIE root *plat- "to spread." Meaning "floor or part of a floor set up as an apartment" is from 1824. Directly from flat (adj.) come the senses "level ground near water" (late 13c.); "a flat surface, the flat part of anything" (1374), and "low shoe" (1834).
late 14c., mudde, "moist, soft earth," cognate with and probably from Middle Low German mudde, Middle Dutch modde "thick mud," from Proto-Germanic *mud- from PIE *(s)meu-/*mu- [Buck], found in many words denoting "wet" or "dirty" (source also of Greek mydos "damp, moisture," Old Irish muad "cloud," Polish muł "slime," Sanskrit mutra- "urine," Avestan muthra- "excrement, filth"); related to German Schmutz "dirt," which also is used for "mud" in roads, etc., to avoid dreck, which originally meant "excrement." Welsh mwd is from English. The older word is fen.
Meaning "lowest or worst of anything" is from 1580s. As a word for "coffee," it is hobo slang from 1925; as a word for "opium" from 1922. Mud-puppy "salamander" is by 1855, American English; the mud-dauber wasp was so called by 1856. The children's mud-pie is attested from 1788. Mud-flat "muddy, low-lying ground near a shore" is by 1779. Mud-room "room for removing wet or muddy footwear" is by 1938.
The expression clear as mud (that is, "not clear at all") is by 1796. To throw or hurl mud "make disgraceful accusations" is from 1762. To say (one's) name is mud and mean "(one) is discredited" is recorded from 1823, from mud in obsolete sense of "a stupid twaddling fellow" (1708). Mud in your eye as a toast is recorded from 1912, American English.
c. 1300, "stretched out (on a surface), prostrate, lying the whole length on the ground;" mid-14c., "level, all in one plane; even, smooth;" of a roof, "low-pitched," from Old Norse flatr "flat," from Proto-Germanic *flata- (source also of Old Saxon flat "flat, shallow," Old High German flaz "flat, level," Old High German flezzi "floor"), from PIE root *plat- "to spread."
From c. 1400 as "without curvature or projection." Sense of "prosaic, dull" is from 1570s, on the notion of "featureless, lacking contrast." Used of drink from c. 1600; of women's bosoms by 1864. Of musical notes from 1590s, because the tone is "lower" than a given or intended pitch. As the B of the modern diatonic scale was the first tone to be so modified, the "flat" sign as well as the "natural" sign in music notation are modified forms of the letter b (rounded or square).
Flat tire or flat tyre is from 1908. Flat-screen (adj.) in reference to television is from 1969 as a potential technology. Flat-earth (adj.) in reference to refusal to accept evidence of a global earth, is from 1876.
1550s, "absolutely, downright;" 1570s, "plainly, positively," from flat (adj.). Flat-out (adv.) "openly, directly" is from 1932, originally in motor racing, picked up in World War II by the airmen; earlier it was a noun meaning "total failure" (1870, U.S. colloquial).