Wow. I don’t often do vanity searches, because nothing ever comes up. But this time I hit the jackpot!
As everyone correctly says, I took the name from Harpo Marx. The poster is reproduced in his autobiography, Harpo Speaks. The name/phrase/whatever has always stuck in my head and I thought it would be unique and memorable as an online alias. Besides, it signals I’m a Marx Brothers fan and people always let me know if a thread breaks out.
And it’s kinda cool in a post-modern sort of way that just like the original Harpo, you can’t hear my voice but you know exactly what I’m thinking. 
I’m a big fan of the book. Dumb cover, though.