गुकारश्चगुणातीतो ।रूपातीतोरुकारकः ॥गुणरूपविहीनत्वाद्गुरुरित्यभिधीयते ॥
In plain words
The sound gu is what has gone beyond the guṇas, and the sound ru is what has gone beyond form. Because he is without guṇa and without form he is called by the name guru.