I forgot to add that one of my friends uses the word "flock" to replace the naughty f-word. Hence, "How the flock are you?", "What the flock" etc
It works rather well, I think, because it still has that satisfyingly venting* short "ck" sound but is comparatively harmless in the meaning department.
* By "venting" - I mean it in the sense that a customer on the phone used it, after ranting and raving for about 15 minutes about things I had no control over, and after not listening to any of my attempts to provide him with facts and clarification of his problem. At the end he said "I just wanted to find someone to whom I could vent my spleen." ("Gee, thanks! Must be my lucky day!")