伊斯 isn't a standalone word — it's a stock prefix in Chinese transliterations of Middle-Eastern, Eastern European, and biblical names:
伊斯兰 (Islam),
伊斯坦布尔 (Istanbul),
伊斯雷尔 (Israel, alongside
以色列),
伊斯特万 (István, the Hungarian name).
伊 covers 'I-/Y-' and
斯 covers '-s-', a pairing that flags 'this is a foreign name' to a Chinese reader.
