Early (1939–1943) cultural and etymological inquiries into 'zoot suit'
American Notes & Queries, volume 3 (July 1943) has this interesting commentary on zoot suit [combined snippets]:
ZOOT SUIT. The New Yorker (June 19, 1943) ignored the more sensational aspects of the "zoot suit" controversy, and commented briefly on the etymology of the term. The word "zoot" is, according to its findings, a corrupt form of suit. And a zoot suit is therefore a "suit worn by a lad who would pronounce it 'zoot.'" Similar "slang rhymes" [cross-reference omitted] indispensable to a description of the outfit—reat pleat ("right pleat," i.e., a good pleat); stuff cuff, etc.—are cited. This same account credits the fad to a Harlem clothier named Lew Eisenstein, whose wife (with the help of a salesman) took in the bottoms of several pairs of pants and created the "pegs" overnight. Another white haberdasher in Harlem, Charlie Kelly, has a variety of documents to back up his claim that he sold the first complete zoot suit in 1937. The New York Times evidence (June 11, 1943, p. 21) carries the zoot suit back to only February, 1940, "at Frierson-McEver's in Gainsville, Ga." Mr. McEver believes that the idea for the zoot suit was original with one Clyde Duncan, for whom he, against his better judgment, had sent the measurements to the Globe Tailoring Company in Chicago. The Times also cites reports that the suit was inspired by authentic Civil War garb worn by Clark Gable as Rhett Butler in Gone With the Wind (which opened in Georgia in December, 1939).
According to Allan Metcalf, From Skedaddle to Selfie: Words of the Generations (2015), the song "A Zoot Suit (for My Sunday Gal)" (written in 1941) matches a boy's desire for a "zoot suit with a reat pleat/And a drape shape, and a stuff cuff" with a girl's desire for a "brown gown with a zop top/And a hip slip, and a laced waist." The song was written by Ray Gilbert and Bob O'Brien—white song composers who presumably were not part of the Harlem "slang rhymes" scene but who had probably been strongly influenced by Duke Ellington's 1941 Jump for Joy show, which, according to Kathy Peiss, Zoot Suit: The Enigmatic Career of an Extreme Style (2011), "brought the zoot suit to center stage as a symbol of expressiveness, fluidity, an freedom."
The February 1940 New York Times article cited in American Notes & Queries (above), as condensed for Negro Digest, volume 1 (1943) [combined snippets], offers the following glancing comment on the origin of the name zoot suit:
A "reat pleat" is merely an exaggerated pleat. It had its origin in assonance, which colors all hep-cat slang. "Zoot suit" is another example. The "V-knot" tie, the zoot chain, the shirt collar, the tight "stuff cuff," the wide, flat hat and the Dutch-toe shoes of the zoot-suiter simply display the hep-cat's tendency toward exaggeration in all things.
The item in The New Yorker (June 19, 1943) [combined snippets] cited by American Notes & Queries (above) says this:
Zoot Lore Last week practically everybody with any pretensions to journalism was ferreting out the origins of the zoot suit, and we ourself were not idle. With some friendly cooperation from the editors of the Amsterdam News, an uptown newspaper published by and for colored people, we got in touch with Lew Eisenstein, proprietor of Lew Pants Store, on 125th Street. Lew, a small, kindly middle-aged man, proved to our satisfaction that the pants of the typical zoot ensemble came original from his atelier. Back in 1934, Lew, who is white, had a lot of about five hundred pairs of pants that he couldn't seem to move. While Lew was away on a business trip, his wife and one of the salesmen decided to see what would happen if they took in the bottoms of several pairs. What happened was that the pants were no sooner in the window than they were snapped up by style-conscious colored boys. "If I'd been there, I wouldn't have let them done it," Lew said of his wife's and salesman's innovation. By the time he got home, though, the style was sweeping along as irresistibly as a tidal wave and he couldn't have done anything to stop it. ...
As for the word "zoot," it is simply a corrupt form of "suit." A zoot suit would be a suit worn by the sort of lad who would pronounce it "zoot." The term might have been of some such incident as a lad going into a store and saying, in all innocence, "I want a zoot." Salesman says, "What?" "A zoot," says the customer,pointing to a rack of peg-topped, drape-shape suits. "Oh, a zoot suit," the salesman says. As for reat pleat, stuff cuff, and the other assonant expressions that go with zoot suit, they're simply the outgrowth of the Harlem trick of slang rhymes. A reat pleat, according to one philologist we met up there, is a right pleat, i.e., good pleat; and that comes from "all reat," which is the rug-cutter's way of saying "all right." That's as far as we're going into the subject.
Earlier than any of these instances are entries from Cab Callaway, "Hepster's Dictionary" (1939):
ZOOT (ADJ.) — exaggerated
ZOOT SUIT (N.) — the ultimate in clothes. The only totally and truly American civilian suit.
If the American Notes & Queries note (above) is correct in arguing that zoot originated as a corruption of suit, it follows that Calloway's claim that zoot means "exaggerated" is evidence that zoot (as a stand-alone adjective) arose after the term zoot suit did, and drew its meaning from the oversize dimensions of the suit.
'Zoot' before the suit
As for pre-1940s mentions of zoot in Google Books search results, all of the ones I checked proved to be OCR garblings, occurrences in misdated publications that were actually from later than 1941, or nonsense syllables, as in this instance from Marie Nelson Lee, "The Tuneless Fiddle," in By Special Request (1921):
Then he dresst up in his best soot
An played, an kep time with his boot—
But the tune want nuthin but "zoot,
Zuggity zoot, zug zoot, zug zoot."
So there is no record—in Google Books search results, anyway—of zoot as a meaningful term prior to the debut of the phrase zoot suit in the late 1930s.