flat (adj.)youdaoicibaDictYouDict
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- (cognates: Old Saxon flat "flat, shallow," Old High German flaz "flat, level," Old English flet (for which see flat (n.)), Old High German flezzi "floor"), from PIE *plat- "to spread" (source of Greek platys "broad, flat;" see plaice (n.)). 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 musical notes from 1590s, because the tone is "lower" than a given or intended pitch; of women's bosoms by 1864. 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.
tuba (n.)youdaoicibaDictYouDict
1852 in reference to a modern, large, low-pitched brass musical instrument, from French tuba, from Latin tuba (plural tubae) "straight bronze war trumpet" (as opposed to the crooked bucina), related to tubus (see tube (n.)).
bourdonyoudaoicibaDictYouDict
"A low-pitched stop in an organ or harmonium, typically a stopped diapason of 16-foot pitch", Middle English (in the sense 'drone of a bagpipe'): from Old French, 'drone', of imitative origin.