[Note: "Thwpp" has many spellings, including "Thwip". My personal preference is to use "Thwpp" for the interjection, and "Thwip" for the verb (because it is easier to conjugate).]
Gbob Waters returned home to Civilization (an apartment in Troy, NY) in a foul mood one day, ready to blow off some steam at the other residents, but not wanting to offend them. So he declared the armchair in the living room to be the Archie Bunker Chair (from "All in the Family"). The idea was that Archie Bunker could sit in that chair and say all manner of rude and insensitive things without anyone taking offense. This would now hold true for anyone sitting in the new Chair.
To continue in Goldenflame's words (I took the liberty of replacing names
One day, I believe Gbob was in the Chair and Jester wanted to slam someone, and so reached out and clasped hands with Gbob. This became known as the Proxy. He then proceeded to slam. There later developed an institution known as the Retroactive Proxy, in which you Proxy after the fact, and once this is completed the recipient of the barbed wit can no longer take offense at it. (They can still beat the shit out of you- they just can't take offense. Kat established this early on). The next step in the evolution of the Chair was the tele-proxy. This occurred when someone was across the room from the chair. The person called out "tele-proxy" or "retroactive tele-proxy" as appropriate and was therefore absolved. The final step was my fault, when I needed to retroactively tele-proxy, pointed to the Chair in the same gesture that Spidey uses when shooting webbing, and said "thwip!"
If you have just said something that you fear will give offense (or are about to, in the case of a preemptive Chair teleproxy), just point your arm with the wrist aimed directly at Troy, NY, palm facing up but bent back (out of the way of the wrist's aim), and optionally with the thumb, index finger and pinky pointed forward and the middle and ring fingers bent in. Say "Thwpp!" Once they release you from the hospital, explain the meaning of the term. Once they release you from the other hospital, thank us for having introduced such a useful concept into your life. Don't you feel more productive already?