1590s, "cloth having a velvet nap on one side," perhaps ultimately from Old English sceacga "rough matted hair or wool," but the word seems to be missing in Middle English. The Old English word is from Proto-Germanic *skagjan (source also of Old Norse skegg, Swedish skägg "beard"), and perhaps related to Old High German scahho "promontory," Old Norse skagi "a cape, headland," with a connecting sense of "jutting out, projecting." Also compare shaw (n.).
The meaning "rough, matted hair, wool, or the like" is from c. 1600. Of a kind of strong tobacco cut in fine shreds, from 1789; of carpets, rugs, etc. made of cloth having a long nap, from 1946.
"tree skin, hard covering of plants," c. 1300, from a Scandinavian source akin to Old Norse börkr "bark," from Proto-Germanic *barkuz, which probably is related to birch and Low German borke. The native word was rind.
<a href="https://www.etymonline.com/word/shagbark">Etymology of shagbark by etymonline</a>
Harper, D. (n.d.). Etymology of shagbark. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/shagbark