This study is concerned with the bai su(背戍) of Koryo period which recorded in Kei Rim Yu Sa(鷄林類事). Results of this research can be summarized as follows : The record of Bai Su(背戍) in Kei Rim Yu Sa(鷄林類事) was correct. It was not a mistake in writing. Thus, this word(背戍) was used by the people of Koryo. The 背戍 of Koryo was related to Aramaic patash and Japanese byets or bats, バツ. It was found that 背戍 of Koryo was very similar to Aramaic patash, legging. It indicates that 背戍 was derived from Aramaic and it was a transliteration of patash. Thus, 背戍 was a borrowed word from Aramaic. Also it was found that 背戍 of Koryo and Japanese byets(ぺツ) or bats(バツ) showed a very close affininty with each other in phonetic value. These words had the same meanings of 襪 one another. It reveals that 背戍 of Koryo and Japanese byets of bats has the same origins. Japanese byets or bats were transliterations of 背戍 of Koryo and they were borrowed words from 背戍 of Koryo.