floss n.

"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," a word of unknown origin. 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.