"rough silk," 1759, of uncertain origin, perhaps from French floche "tuft of wool" (16c.), from Old French floc "tuft, lock," from Latin floccus "tuft of wool" (see flock, n.2). Or from a dialectal survival of an unrecorded Old English or Old Norse word from the root of fleece, n.. Compare the surname Flossmonger, attested 1314, which might represent a direct borrowing from Scandinavian or Low German. In "The Mill on the Floss" the word is the proper name of a fictitious river in the English Midlands. Meaning "fine silk thread" is from 1871, short for floss silk (1759). Dental floss is from 1872; the verb floss in reference to use of it is from 1909. Related: Flossed; flossing.