comes from the same source as recipe, I think. It's the fault of Dr Johnson and his dictionary, which did a great deal to fix English spelling in the 1700s. The P was already not pronounced, but he still included it - and yet he left it out of deceit.
Ooh folks, just realised that this thread is going rapidly down the pan ... toilet humour is just not funny! (Well okay then, sometimes it is, but not this time).
I didn't 'twig' about the toilet connections when I first posted! Ewwwwwwww!!!