Would it be a mistake to use the word "gay" in business or academical writings? [duplicate]

This is a post by NiceTryDad on instagram.

Son: "What does the word "gay" mean?"
Me: "It means happy."
Son: "Oh, are you gay?"
Me: "No, son, I have a wife."

Later then, I googled up the meaning of the word "gay". And I found that it has some meanings in its older use, like having or showing a merry, lively mood, and also bright or showy. And also, this site explains:

Gay meaning ‘homosexual’ became established in the 1960s as the term preferred by homosexual men to describe themselves. It is now the standard accepted term throughout the English-speaking world. As a result, the centuries-old other senses of gay meaning either ‘carefree’ or ‘bright and showy’ have more or less dropped out of natural use. The word gay cannot be readily used today in these older senses without arousing a sense of double entendre, despite concerted attempts by some to keep them alive. Gay in its modern sense typically refers to men (lesbian being the standard term for homosexual women) but in some contexts it can be used of both men and women

I know that that post was just a joke, but I'm wondering if I can use the word gay to indicate that someone's happy in business or academical writings. Since people would always refer it to its modern sense (homosexuality).

You ask:

I know that that was a joke, but is it still appropriate to say gay to refer that someone's happy or merry these days? Like in writings or something.

When words have multiple meanings, each individual meaning can be used appropriately, given suitable context. The tricky part is to make sure that the context constrains the word to only the meaning or meanings that are intended.

For example, saying "the yolk has set" would almost certainly be understood to mean that it has hardened, particularly if the context is cooking. Saying "the sun has set" would usually not convey the same sense of the word set (i.e. that the sun has 'hardened') - it would instead almost certainly be read as talking about a period of time in the day-night cycle.

Now, when a word has undergone a semantic shift, the context must work a lot harder to bring out the older meaning of the term. For example, awful no longer carries the sense of "full of awe", so one can't just speak of an "awful experience"; one might have to speak of an "ecstatic, blissful, or as the ancients might say, awful experience".

In the case of the word gay, popular usage these days would tend to default to the homosexual interpretation, but it's not a semantic shift. Both senses are understood, but since the happy sense of the word is no longer dominant, it requires more from the context.

So in short, yes, it's still appropriate to use the word gay in the sense of happy, so long as it's presented in appropriate context.

I'm going to answer this but please don't anyone get on my case because I have no opinion on the matter. I don't want to offend anyone but I do want to offer an honest answer.

For most people in the US, or native English speakers, the first thing that will come to mind when they hear "gay" will be "homosexual". However, it's perfectly okay to use the word "gay" to mean "merry, lively, and/or happy". When it is used in that way it is more often in writing versus in speech.

The bottom line is if you walk up to many people and say, "I'm feeling very gay today.", they are going to think you mean you are homosexual. The mature ones will likely understand what you mean from your context and attitude. The immature ones will snicker and laugh.