What is "yinxi"

Yinxi, formerly romanized as Yin-hsi, was a legendary figure of Zhou China. He was said to have been a guard at the western gate of the Zhou capital Chengzhou (present-day Luoyang) or, alternatively, at the western pass out of the Luo– Yi valley. His own wisdom caused him to halt Laozi on his way through the gate and, supposedly, he successfully importuned the sage to compose the Tao Te Ching before permitting him to pass.