It's an extremely outdated phrase meaning something like: 'He's so funny!'. Nobody has said this since about 1950. You should not try to use it.

You don't take criticism well, do you? That's my experience with you, at least. You need to re-read the OED entry. And I can put the same question back to you, LL. "Nobody has said this since about 1950." Your source? Honestly, if you make a mistake, just deal with it. At minimum you need to clarify that you are *only* speaking about UK English. Try that from now on. It will be more helpful to the English learners here, and especially much more helpful than schoolmarmy advice like, "You should not try to use it." Shame on you for mucking up Ishita's post like that.

As a middle-aged American, I’ve never heard or used the phrase either. But A google books ngram for a similar phrase “you’re a caution,” shows it making a resurgence. https://books.google.com/ngrams/graph?smoothing=3&corpus=26&year_end=2019&year_start=1800&content=You+are+a+caution&direct_url=t1%3B%2CYou%20are%20a%20caution%3B%2Cc0 It still doesn’t seem to be a widely used phrase, but maybe we’ll be hearing more of it.

Thank you, Angela.

So, I'm a middle aged person from the western US and I've never heard it. Also, if you did want "proof" here's the google n-gram: https://books.google.com/ngrams/graph?smoothing=3&year_end=2019&case_insensitive=on&year_start=1800&corpus=26&content=%22a+caution%22&direct_url=t4%3B%2C%22%20a%20caution%20%22%3B%2Cc0%3B%2Cs0%3B%3B%22%20a%20caution%20%22%3B%2Cc0%3B%3B%22%20A%20Caution%20%22%3B%2Cc0#t4%3B%2C%22%20a%20caution%20%22%3B%2Cc0%3B%2Cs0%3B%3B%22%20a%20caution%20%22%3B%2Cc0%3B%3B%22%20A%20Caution%20%22%3B%2Cc0 it looks pretty clear that the phrase was used *a lot* in the late 1800s, a little in the early 1900s, and barely at all since about 1980 (40 years ago). There's nothing incorrect with LL's response to Ishita.