Does anyone know where “negatory” came from?  I said it to my son today and he asked if I got it from Toy Story.  I said no, it’s from 1970’s CB/trucker slang.  Then of course I realized that I really had no idea if the word came from CB/trucker slang or if it predated the CB craze of the 1970’s.  Does anybody know for sure?

etymonline says:

negatory (adj.) Look up negatory at Dictionary.com
“expressing negation,” 1570s, from M.Fr. negatoire, from M.L. negatorius “negative,” from L. negatus (see negation). In the sense “no” it is U.S. Air Force slang from the early 1950s.
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A little more from OED, mentioning the CB connection.

negatory, int. and n.

slang (orig. U.S. Air Force).

A. int.

Esp. among Citizens’ Band radio users: ‘no’, ‘negative’.

1955 Amer. Speech 30 118 Negative; Negatory;‥, I refuse; I disagree; no (in answer to a question).
1976 Daily Tel. 16 July (Colour Suppl.) 10/1 A CBer says‥‘negatory’ for ‘No’.
1992 MacUser Nov. 32/2 Surely you’ll have all the bases covered won’t you? Negatory.
2001 Washington Post (Nexis) 1 July w14 ‘Negatory, Red Bird, we can’t help,’ a trucker radioed apologetically after our new plea.

One reason negatory may have gained popularity in CB radio is that affirmative and negative share the same concluding phoneme and can be confused over the radio, particularly when the signal is weak, staticky, or when the speaker starts talking before pressing the push-to-talk button.

Reminds me of nine/niner in aviation with various explanations here.
I have just watched Toy Story 3 again and the toy pig does say negatory.

My grandmother was a telephone operator in the early 1920’s and she said they were taught to say 9 like “nyan” (rhymes with Ryan).

Edited to add rhyme note.

Yes, Ernestine (Lily Tomlin) used that pronunciation.  I found a description here:

Of great fascination to me is the elocution training given to operators as part of their rigorous training. Yellins explains: “They were taught to draw out certain words for clarity because early telephone lines were noisy. So the word please would be pronounced “pleeeyaz”. The number nine became “niyun” and the word line turned into “liyun.” What I always thought of to be a regional “tick” (or stereotype) was actually borne out of necessity to overcome the sound issues of early crude telephony lines.

CB and other two-way radio users said “niner”, for a while.

EDIT: do cats have nyan lives?