I read the 11 Nov. 1978 panel of Garfield, in which the phrase "have got it knocked" is used. Transcript:
[Garfield is in bed but mobile]
Garfield: Oh-oh. I feel a nap attack coming on
[Garfield suddenly assumes an immobile, sleeping position]
[Garfield peeks from under covers]
"Turtles have got it knocked"
On my own, I thought this was another phrase for "have (got) it (all) figured out," by which I mean something like "one has established an appealing but possibly theoretical model for achieving some end." Citing AHD 4th ed., TFD provides this definition for have it knocked:
have it knocked Slang
To be certain of success: "He knew he had it knocked after he saw a rough cut of Chinatown" (Time).
Strikingly similar definitions can be found for the related phrases have it made (Dictionary.com based on Random House Dictionary: to be assured or confident of success) and have it made in the shade (TFD citing McGraw-Hill Dictionary of American Idioms and Phrasal Verbs 2002: to have succeeded; to be set for life).
For each but have it knocked I can find reasons to satisfy my curiosity about why its ending might be chosen:
- Figured out is an idiom by itself meaning to begin to comprehend someone or something
- Made can be used figuratively to mean assured of success or fortune: a made man.
- Made in the shade rhymes with the above, retaining its meaning while emphasizing the subject's present comfort
I suspect knocked may come from sports, where knock has a sense of finality and success: In boxing, a knock-out means a win; likewise in baseball, knocking a ball out of the park is used to indicate a good hit. But it seems odd to me that out would be omitted, because in the mentioned phrases it emphasizes the action's success and finality.
To measure the likelihood of these guesses, I am interested in the origin of the phrase have it knocked. I could not find a date like the 1955 Etymology.com provides for have it made. Derivations, if found, are also welcome.