Word Origin & History
O.E. meos "moss," related to mos "bog," from P.Gmc. *musan (cf. O.H.G. mios, Ger. Moos), also in part from O.N. mosi "moss, bog," and M.L. mossa "moss," from the same Gmc. source, from PIE *meus- (cf. L. muscus "moss," Lith. musai "mold, mildew," O.C.S. muchu "moss"), from base *meu- "moist, marsh."
All the Gmc. languages have the word in both senses, which is natural since moss is the characteristic plant of boggy places. It is impossible to say which sense is original.
"Selden Moseþ þe Marbelston þat men ofte treden." ["Piers Plowman," 1362]
Scott (1805) revived 17c. moss-trooper "freebooter infesting Scottish border marshes." Mossback "conservative" is 1878, originally of poor whites from Carolina, originally (1872) in ref. to those who hid out to avoid service in the Confederate army (and would have stayed out till the moss grew on their backs).