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
Elements
Word Gloss flâd “skin” rîf “bark”
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).