Falathar contains Sindarin falas ("shore, line of surf"). The ending is not explained, but a relationship to thar- "across, over, beyond" is not inconceivable.
Sindarin
falathar
masculine name. Falathar
Elements
Word Gloss falas “beach, shore, strand, surf(line)” ?
Falathar
Falathar
A companion of Eärendil (S/248). The meaning of this name is unclear, but it appears to contain a variant of falas “shore” (SA/falas).
Conceptual Development: In Silmarillion drafts from the 1930s, his name was also N. Falathar (LR/324).