When our middle child Heidi was just learning to talk (age 2 or so) she kept asking for "seekadoo". We had no idea what she wanted at first. But it seemed by the way she was asking that she wanted something to drink. After a bit of trial and error we finally figured out that she was asking for a carbonated beverage (soda, pop, cola). We have no idea why she was calling pop seekadoo, as the words aren't even close to similar. But that's what she wanted.
I have liked to make up silly nonsense words since I was a child, and this new word sounded cool. So I picked up the term and began using it as well. Everyone else in the family caught on, so now seekdoo is the word we use for pop. Most of our friends and family know what we're talking about as well when referring to seekadoo. Of course we've gotten some strange looks from servers at restaurants, but for the most part it's all fun.
Spelling - the cool thing about creating nonsense words is that you can spell them however you want. I personally like the spelling 'sicadoo', but it seems like spelling it that way would more likely result in a mispronunciation by people who are unfamiliar with the word. So seekadoo it is.
I'm thirsty. Time to have a seekadoo.