Rainer \ra(i)-ner\ as a boy's name is pronounced RAY-ner. It is of Old German origin, and the meaning of Rainer is "deciding warrior". Variant of Raynor. Related surnames are occasionally used as given names. Mount Rainier is an important landmark named for a British naval officer, but the Indian name was Tacoma or Tahoma, meaning "snowy peak". Modern use of Rainier (ray-NEER) may be influenced by Prince Rainier of Monaco. Poet Rainer Maria Rilke.