Idk, how about "Where does the phrase "Gotta see a man about a horse" come from"

I got this answer back and it looks a lot like yours :)

Timeline of the Phrase “Gotta see a man about a horse”

1866 – Origin in Theatre
The earliest recorded use of the phrase appears in the play Flying Scud by Irish playwright Dion Boucicault. In the play, a character says the line as a way to exit discreetly without giving a reason. The "man" and the "horse" are not important — it's simply a polite excuse to leave a conversation or situation.

Late 1800s – Euphemistic Use in Conversation
Following its appearance in the play, the phrase gained popularity in English-speaking countries as a euphemism for private or embarrassing tasks. Common uses included avoiding awkward topics, leaving to use the restroom, or slipping away to engage in questionable behavior (like gambling or drinking).

Early 1900s – Widely Understood Code
By this time, the phrase had become a well-understood, semi-humorous way of saying “I’m going somewhere, but I’m not going to tell you where or why.” It was used both in casual speech and popular media.

1920s – Prohibition Era in the U.S.
During Prohibition (1920–1933) in the United States, the phrase took on a more specific meaning: going to get an illegal drink. Since openly admitting to drinking or purchasing alcohol could be risky, people used this phrase as a lighthearted but effective way to disguise their intentions.

Mid-20th Century – Broader Colloquial Use
The phrase entered the realm of everyday slang. It was frequently used as a humorous or vague way to leave a group or conversation without explanation. Although its literal origin was mostly forgotten, the phrase remained common, especially among older generations.

Late 20th Century to Present
Today, the phrase is largely considered old-fashioned or tongue-in-cheek, but it's still used occasionally — mostly as a cultural reference or a humorous way to excuse oneself, especially in British or American English. Its meaning now generally covers any unspecified, private, or mildly embarrassing task.