"finer portion of ground grain," mid-13c., from flower (n.), and maintaining its older spelling, on the notion of flour as the "finest part" of meal, perhaps as the flower is the finest part of the plant or the fairest plant of the field (compare French fleur de farine), as distinguished from the coarser parts (meal (n.2)). Old French flor also meant both "a flower, blossom" and "meal, fine flour." The English word also was spelled flower until flour became the accepted form c. 1830 to end confusion. Flour-knave "miller's helper" is from c. 1300.
"to sprinkle with flour," 1650s, from flour (n.). Meaning "convert (wheat) into flour" is from 1828. Related: Floured; flouring.