bamboo
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竺 centers on bamboo as a plant or material. It is a specific kanji for bamboo, often used in names or classical contexts.
India
guinea pig (any of five guinea pig species, esp. Cavia porcellus)
shiny guinea pig (Cavia fulgida)
Brazilian guinea pig (Cavia aperea)
montane guinea pig (Cavia tschudii)
China and India; far away
cotton sheeting
gagaku of Indian origin
wandering lordless samurai
dahlia (Dahlia pinnata)
Jersey stitch (knitware)
geranium (Pelargonium inquinans)
vertical-striped cardinalfish (Apogon lineatus); Indian perch; coral fish
Indian studies; Buddhist studies
竺 is composed of 竹 (bamboo) on top and 二 (two) below. The exact historical origin is uncertain; it may have originally depicted bamboo stalks or been a phonetic loan.
Two bamboo stalks (竹 on top, 二 below) standing together: 竺 is bamboo.
For ジク, imagine a thick bamboo stalk that you can jiggle: jiggle -> ジク, and the bamboo bends but doesn't break.