Tolkien wrote that Skinbark is the Common Speech (or English) translation of Fladrif. It appears like Fladrif is intended to be Sindarin; the name is mentioned together with the Sindarin name Nindalf as acquiring the /v/ sound by the final -f. Didier Willis has suggested that Fladrif is composed of the otherwise unattested words flâd ("skin") and rîf ("bark").
Sindarin
fladrif
masculine name. Skinbark
An Ent name glossed “Skinbark” (LotR/474). It is unclear what the exact meaning of its elements are, though David Salo suggested that flâd is “skin” and rîf is “bark” (GS/255, 282).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name first appeared as N. Fladrib, quickly revised to N. Fladrif (TI/412).