A preposition or prefix for “across, over” or most properly “athwart” (PE17/34). In a rejected note Tolkien said that this word meant “across, properly not of position across or beyond but of act of crossing, or act[ual] position ‘athwart’ ... in names of districts, reached by crossing an obstacle, it became used like ‘Trans-’ (as ‘Transvaal’), as in Thargelion [Land beyond Gelion]” (PE17/34). Despite this note’s rejection I think this explanation remains valid. Thus thar is “across” in the sense “crossing” or “laying across” rather than “on the other side”. It was derived from the root √THAR of similar meaning (PE17/14; Ety/THAR).
Conceptual Development: The Gnomish Lexicon of the 1910s had preposition and adverb G. adr “across, athwart” as well as adjective G. {aðra >> athra >>} adr(a) meaning either “lying athwart” or “situated on far side” (GL/17). These Gnomish words were related to ᴱQ. arta, and thus clearly derived from the early root ᴱ√TARA having to do with crossing (QL/89).
_ pref. _across, over, properly 'athwart'. Original S. form þara-. See also the rejected glose in PE17:34.