Over on my own journal, I'm running a theme week of words beginning with pt-, which I'm continuing here with the joker in the pack: my other words get that unusual consonant combination naturally by descending from Greek words, where they were used to pronouncing it and so did so, this one does not. It's actually the Scots Gaelic name for the bird, originally tàrmachan (a diminutive of tàrmach, croaker). The p- was added in 1684 by Robert Sibbald. who apparently either thought it was originally Greek and he was restoring its proper spelling or he wanted to dress it up and make it look like it was originally Greek. In either case, unlike actually Greek words such as pterodactyl, the p is entirely silent. As a usage example, I borrow a joke from Pogo, where I first met the word.
Most of the banquet was good, but the ptarmigan was ptough.