regeneration - When was the phrase "Timey Wimey" first invented by the Doctor?

The War Doctor never claims to have not heard the phrase. He's just disgusted that his two ostensibly older selves are still using it, since it's clearly a child's phrase. The implication has always been (from its very structure) that it's the sort of thing that Time Lord children learn in school and eventually outgrow.

In general, the implication of Day of the Doctor is that, for all that the stories of the series are somewhat more mature in their approach much of the time, the Doctor has somewhat deliberately reverted. The Doctor has always had both a child-like and a child-ish streak to him, but 10 and 11 have brought the child-like streak out much more strongly, while the War Doctor was Very Serious and hence, very grown-up.

In some ways, the War Doctor reminding the Doctor that being a grown-up should not be a bad thing is also a set-up for the new, more mature (age-wise, anyway -- we'll see about his personality) Doctor debuting in August...