Tenjiku (天竺) is the Japanese word, which derives from Chinese word Tianzhu(天竺) commonly used in reference to pre-modern India.
The Chinese surname Zhu 竺, which originally meant "a kind of bamboo" and was later used for "India (abbreviating Tianzhu 天竺)" and "Buddhism", was adopted by many early Buddhist monks, such as the polyglot translator Zhu Fahu 竺法護 or Dharmarakṣa (c. 230-316).