Origin of phrase "Making Winkies" as euphemism for urinating

Factual Questions
1

A certain doper who shall remain nameless recently used the term “making winkies” as a euphemism for taking a leak. I had never heard the term before, despite growing up in a home with three older brothers all of whom had a colorful vocabulary for such things. The only thing connection I could come up with is that the Winkies in “The Wonderful Wizard of Oz” (the book, not the movie) were yellow.

A google search and a board search revealed nothing of any use. Can anyone clue me in on the origin of this phrase (presuming, of course, the above unnamed doper didn’t just make it up)?

2

It may also be a natural evolution of “tinkle.”

“Tinkywink” is a pretty common thing to call a toddler, esp. in the britain.

“Tink” and “Wink” seem to be natural substititions, too… Rudyard Kipling has a fun poem called the Song of the Banjo, the first line of the chorus of which changes between

“With my tinky tinky tinky ta” and “With my willy-nilly winky winky pa!” etc.
The only part I remember with any clarity is

(Not that that part has any particular relevance, but I love it.)