Do you know that we can trace the silent P to Greece? Most English words with a silent P come from the Greek. Many, like psycho, begin with the Greek letter Ψ (psi), which "indicates the combination /ps/ (like in English "lapse")[... but in English] pronunciation is usually simplified to /s/." (See, in Greece, they actually pronounce the p, but we are apparently too lazy for that.)
According to Online Etymology, pneumonia, pterodactyl, psalm, ptomaine, and many others (all the ones I could think of) have their roots in Greek with a side trip through Latin.
So the next time you want to spell pseudonym, you can thank the Greeks for that extra P.