mossyoudaoicibaDictYouDict[moss 词源字典]
moss: [OE] The prehistoric Germanic ancestor of moss was *musam. This had two distinct meanings: ‘swamp’ and ‘moss’. It is not altogether clear which was primary, but it seems more probable than not that ‘moss’ (a plant which frequents damp places) was derived from ‘swamp’. The only meaning recorded for its Old English descendant mos was ‘swamp’ (which survives in place-names), but no doubt ‘moss’ (not evidenced before the 14th century) was current too.

Words from the same ultimate source to have found their way into English include mire [14] (borrowed from Old Norse mýrr ‘swamp’), mousse [19] (borrowed from French, which got it from Middle Low German mos ‘moss’), and litmus [16] (whose Old Norse source litmosi meant literally ‘dye-moss’ – litmus is a dye extracted from lichens).
=> litmus, mire, mousse[moss etymology, moss origin, 英语词源]
moss (n.)youdaoicibaDictYouDict
Old English meos "moss," related to mos "bog," from Proto-Germanic *musan (cognates: Old High German mios, Danish mos, German Moos), also in part from Old Norse mosi "moss, bog," and Medieval Latin mossa "moss," from the same Germanic source, from PIE *meus- "damp," with derivatives referring to swamps and swamp vegetation (cognates: Latin muscus "moss," Lithuanian musai "mold, mildew," Old Church Slavonic muchu "moss").
Selden Moseþ þe Marbelston þat men ofte treden. ["Piers Plowman," 1362]
All the Germanic languages have the word in both senses, which is natural because moss is the characteristic plant of boggy places. It is impossible to say which sense is original. Scott (1805) revived 17c. moss-trooper "freebooter infesting Scottish border marshes."