I know Simon, and Simon knows me

Explanation

the Telchines being malicious by nature (for sprinkling the earth with the water of Styx they made it barren), there were two of their leaders, Simon and Nicon. Simon prevailed, being the most evil-natured, so that he wiped out Nicon's reputation. So in proverbial use only Simon is named. The proverb is said of those who know one another in their wickedness.