1851, from flat (adj.), which was used from late 14c. of plates, dishes, saucers in a sense "shallow; smooth-surfaced" + ware (n.). Originally as distinguished from hollow ware; U.S. sense of "domestic cutlery" recorded by 1895.
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.
"manufactured goods, goods for sale," Old English waru "article of merchandise," also "protection, guard," hence probably originally "object of care, that which is kept in custody," from Proto-Germanic *waro (source also of Swedish vara, Danish vare, Old Frisian were, Middle Dutch were, Dutch waar, Middle High German, German ware "goods"), from PIE root *wer- (3) "perceive, watch out for."
Usually wares, except in compounds such as hardware, earthenware, etc. Lady ware was a jocular 17c. euphemism for "a woman's private parts" (but sometimes also "male sex organs"), and Middle English had ape-ware "deceptive or false ware; tricks" (mid-13c.).
<a href="https://www.etymonline.com/word/flatware">Etymology of flatware by etymonline</a>
Harper, D. (n.d.). Etymology of flatware. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/flatware