My late father was a veritable fountain of expressions, many of which I've picked up. It sometimes annoys me when I realize that I'm parrotting him. Other times, I enjoy it.
When we were young, he often told us that he couldn't say "right," he could only say "left." Then we'd try to trick him into saying "right." He also told us that he couldn't say "hospital," he could only say "hospistal."
The one expression that my father owned, because I never heard anyone else say it (unlike many, many of the expressions in this thread) was, when someone would say, "You're kidding," he would say, "No, I'm wearing my uncle's suit." I used to ask him what this meant. Finally, he told me that kidding was an old, slang term for being pregnant, and your uncle's suit was too big, making you look fat and pregnant.
I often use the expression, but it's a pain to try and explain it. So, instead, I use an expression taught to me by DD6 (no idea where she got it) -- "It's just an expression, like 'it's raining cats and dogs.'"
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When exasperated she would say "Well, I swan" I guess that is too avoid cursing, but she did that occasionally too 
She also gave us a lot of "Oh, Lordy!"
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