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.
Old English wurm, variant of wyrm "serpent, snake, dragon, reptile," also in later Old English "earthworm," from Proto-Germanic *wurmiz (source also of Old Saxon, Old High German, German wurm, Old Frisian and Dutch worm, Old Norse ormr, Gothic waurms "serpent, worm"), from PIE *wrmi- "worm" (source also of Greek rhomos, Latin vermis "worm," Old Russian vermie "insects," Lithuanian varmas "insect, gnat"), from PIE *wrmi- "worm," from root *wer- (2) "to turn, bend."
The ancient category of these was much more extensive than the modern, scientific, one and included serpents, scorpions, maggots, and the supposed causes of certain diseases. For substitution of -o- for -u-, see come. As an insult meaning "abject, miserable person" it dates from Old English. Worms "any disease arising from the presence of parasitic worms" is from late Old English. Can of worms figurative for "difficult problem" is from 1951, from the literal can of worms a fisherman might bring with him, on the image of something all tangled up.
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Definitions of flatworm
parasitic or free-living worms having a flattened body;