Where did the "juices" in "creative juices" come from?

Solution 1:

An Ngram of the term shows its birth as a "stock phrase" only dates back to the 1960s, but the words are found in print a century earlier and the first examples of the phrase being used to mean motivating, inspiring, or enabling forces or factors go back to the 1930s. I'll give you one example of each.

I found creative juices in print back to this 1846 translation (p. 182, scroll down) of Kalevala, a Finnish epic poem that recounts a mythical story of the cursed birth of steel:

I don't think this reference is incidental. The same section of this poem, with the same translation, was reprinted several times in the writings of Lafcadio Hearn at the turn of the century and as late as 1922. While not the figurative use of the phrase we have today, I think there's a good chance its occurrence in his writings led to its familiarity and adaptation in the 1930s.

The first use of it I could find in print with its modern connotation was from this 1936 article in The Delineator:

[...] and played and replayed and recorded and wrote down their early songs. Their creative juices dried up. Only a few of the original players stuck by their guns, among them notably the men mentioned in this article. They continued, in honky-tonks, dives and dance halls, to play as they felt and feel as they played.

I wasn't able to get a more complete quote because of the limitations of Google's Snippet view, but it's clear the article is referring to the figurative creative juices of musicians.

One later 1930s reference of note is found in the intro to the 1939 screenplay of Margaret Mitchell's Gone with the Wind.

Solution 2:

It could possibly have to do with the use of juice as a slang term for power or electricity. I found several quotes (all from the OED) that use juice in common circumstances when something is running or powered:

• 1896: “Now we know what a blessing the trolley is—when the juice isn't turned off.”—Boston Herald, 25 December, page 4/5
• 1908: “Electric cars all stopped; no juice, no nothing, everything stopped.”—Saturday Evening Post, 18 July, page 10/2
• 1928: "The juice was turned off, and Vanzetti was officially pronounced dead.”—Boston (1929) by Upton Sinclair, xxiv. page 724
• 1948: “Turn on the juice to this elevator, and do it now.”—Great Falls Tribune (Montana), 18 September, page 12/1
• 1997: “Electric cars have a range of about 80 miles before they run out of juice.”—Daily News (Los Angeles), 9 October

Solution 3:

Juice probably refers to cerebrospinal fluid around and inside the brain.

Solution 4:

For what it's worth.

In French you can hear "Jus de crane" (skull juice). This attempts to convey the idea of "squeezing ideas" out of your brain.

I think that's close to the metaphor here: squeezing out creativity.

Same if you say "Je suis sec, je sèche..." ("I'm dry...").

Solution 5:

Creative is very new, in the literary sense, (Wordsworth C19) so it's just somebody being poetic (and creative)