[Albert Moulton]

Another typical example is an overpair vs bottom two pair.

For example, let's say I raise pf with AA. I'm called by 67s. The flop is Q67r, and (for whatever reason) we both end up all in on the flop. If the board pairs any card other than 6 or 7 (i.e. a Q on the turn), then the 67s hand is "counterfeited."

I have no idea about the etymology other than to speculate that a counterfit dollar bill is worthless. And when the board pairs giving the higher pair a higher two pair it renders the lower two pair as worthless as a counterfit dollar bill.